So the basic work surface is completed, it is reasonably sturdy, the back rail is missing its gussets which I will cut and glue on today. But there were a few important errors.
The frame can be assembled with the legs in the vertical position, as shown here, or as a shorter platform, which is position I will be using for the dinghy once I get that far. But the plans also included a pair of short rails and in the cutting / marking of making those I made a fatal error.
The length of the plywood I had to work with was 63″ plus some stray fractions left over after saw kerfs, and I needed a 30″ rail. But there was no reason why I could not make it slightly longer, say 31.5″ which would be a neat halving of the actual length if I rounded off to exactly 63″. So I measured out the one rail to exactly 31.5″ and cut it, with a note to myself when I got to the rail-assembling step I would trim the other off precisely.
And you all know what happened next: I never did trim that rail properly. So the slots on one short rail are about .75″ an inch further apart than the other. The short rails are useless, unless I make 1 new one to exactly match one of the existing ones.
Of course it was when I was berating myself about this error that I realized I had been an idiot in another way, too. I marked and cut each of the four rails separately. If instead I had clamped the two matching-sized rails together I could have marked each pair, drilling and cutting the slots at the same time, and thus creating exactly paired rails. And, incidentally, this would have clearly reminded me to trim the short rails to a matching length.
More lessons which I am so much happier (re)learning on this project than, say, something involving rare tropical hardwoods. I can go out and purchase a 1/4 sheet of plywood and another cheap pine stud to make two new short rails for about $20 (but I will need another tube of glue, which will cost $15!)