Here is a photo Charlie sent me:
As you can see he definitely has rear legs installed on the seat. I don't think I would put all that weight on the plywood companionway edge--it can probably support the weight, but just in case, I think it needs the back legs too, especially if you ever stand on it to raise the mast the way I plan to. Charlie made his seat so the legs fold up, so did not add steps, but he also has a low step-stool down on the cabin floor. I am making mine rigid so I can add the steps. It will take up more room but I don't think that will be a problem (and I don't see any reason why you can't put a pad on my seat design--in fact I plan to at least put a floatation cushion on it).
Charlie also sent me this picture:
His folding design is quite a bit different from my rigid design. I think there are advantages and disadvantages to each.