Got the two pieces of ply scarfed together today. I'd liked to have gotten more that that done, but I was able to move enough stuff around while the epoxy was curing to be able to move the full length skiff into the garage for the night. Plus, I extracted my table saw from the garage so I can use it tomorrow. (That was a major accomplishment.)
In the past, I used an epoxy and fiberglass butt joint to make a long piece of plywood out of two shorter ones. It's a good joint, easy to do, and doesn't look bad at all. To me, it's better looking than using a butt block, sometimes called a "hog-piece. But that does the job, too. I have made scarf joints, but with solid wood, not ply. I've scarfed rub rails together, a pair of paddles to make a double-ended paddle, and some chair rails when re-doing the bedroom.
So, a scarf joint for plywood. It is do-able, even for a complete novice, but you really should take some scrap ply and practice. It doesn't take long to "get the touch" so you can do it right, but definitely get some practice FIRST. You'll see why, in a moment.
Normally, a scarf joint would be an 8:1 joint. If the ply is 1/4" then the joint should be 2" long. I point that out because this design calls for a "short" scarf, 4:1, or a 1" joint. The 8:1, or even 12:1 was done before epoxy came along and the theory is that the much stronger glue should let you use a lesser ration, like the 4:1. However, I don't think anyone has really tested this theory. Why Mr. Simmons decided to go with the 4:1, I don't know. But apparently it worked for him and I am building as close to his method as I can.
Here is how it goes:
You first decide on which sides are to be up, usually the best side, and which side you want down, (on the bottom. For the 4:1 joint, in 1/4" ply, mark off one inch on each piece. For this demo, I used a pair of scrap cut-offs:

The above left pic, the "T" means the top side, the line is one inch from the end, the part to be planed, or sanded, tapering from the line to the bottom at the end.
The above right pic shows both pieces from the side, with the wood to be removed blacked out.

You will be removing wood from BOTH pieces at the same time. And doing it this way, an unevenness on will, will be reflected in the other, and will sort of cancel each other out. (More-or-less, anyway.) In the above left pic, you "swing" the one piece down, as if the two were hinged together. Then, as seen on the above pic on the right, the swing is completed, one piece on top of the other.

Above, you see where the top piece has been slid back, so that the end of it is even with the 1" line on the bottom of the lower piece. Sand or plane the black parts away as shown above, reverse that swing you did, and you should get a nice fit, ready for gluing.
So much for theory, now for the reality:

Can't see in that pic, but the wood is sitting on a pair of planks, the ends even with the end of the lower ply sheet. C clamps have been placed to keep them lined up, you'll notice some scraps pieces being used with the clamps to protect the ply. The belt sander (80 grit to remove wood quickly may be a bit too much,) shows the angle to be sanded.

As you sand (or plane,) you'll be exposing the different layers in the ply, they will each show a different color. What you are looking for is to see these different layers showing as ribbons or bands, each the same width and with straight edges. That tells you the angle is correct. Really pay attention to that!
This is what it looks like when you aren't paying close enough attention, maybe because it is extremely HOT outside, you are sweating all over your work, and you're in a hurry because you're just this side of a heat stroke. (Or any other excuse you can think of.)
Truthfully, it should look a LOT better than this. I shouldn't have started this during the hottest part of the day. I should know better. And some practice before hand is something I should have done, but didn't. Here is where some thickened epoxy will save the day, once again. (Hopefully.)

I did spend a little time cleaning those edges up. Totally unnecessary if you do it right the first time. Then I move the lower piece in position, and slid a large piece of scrap ply underneath the joint area. A sheet of clear plastic was placed underneath the joint, wide enough to keep any epoxy from gluing the ply to the wood below. I then mixed up some epoxy without any thickener, to both pieces to be joined. While that was starting to kick off. I positioned the forward sheet and drove a couple of small nails through it into the wood below, to hold it in place. Then I mixed up some more epoxy, this time with wood flour (much finer that sawdust,) to a consistency like a runny baby food. (?)
This mixture was then applied to both pieces. Positioning the two pieces is awkward, on a good day. Even more so when working alone. But you do have a little time, so use it, don't rush things. Once the parts are together as close as you can get, and you've looked at the whole thing from both ends, to make sure it looks good and straight, another set of nails should be driven through the other sheet into the wood below, to prevent it from sliding away from the joint.
A second sheet of plastic is placed across the joint, so it wont stick to the wide, flat piece of wood that goes on next. And on top of that, goes some weights. Do NOT get carried away with the weights! Just enough to press that top piece of wood down on the joint. Too much weight will squeeze out the epoxy, resulting in a glue-starved joint that will fail. I used three jugs that I filled with water for this.
Now, nothing to do until the glue has fully cured. Once that happens, you have one long sheet of ply. When properly done, if it breaks, it will break anywhere
other than that scarf joint. If you've never done a scarf joint and never intend to, I'm sure this post will bore you to tears! I apologize for that. But I went into detail, as I was asked about how it was done.
That's it for me. I'll be back at it in the AM, starting on figuring out how to shape and attach the cypress plank sides.
Mike S.
Spring Hill, FL