You're welcome, Charlie. Yes, using three pieces (one large piece with two small "wings") is the key, as it allows the seat back to conform to the curvature of the front shell when lowered. As I said earlier, it's really a well-thought-out design. The only real weak point is the glassed-in ledge under the window that supports the front of the bunk. Over time, leaks
in the front window can cause the wood inside to deteriorate. Have you made sure yours is sound? (I'm assuming here that Casita
does the same as Scamp
You might do a search or post a request for templates for the plywood and cushions, here and on the Casita forum. It could save some time.
BTW- have you considered looking into ordering cushion covers from Casita, or maybe even Scamp
? I know Scamp's are very reasonable compared to custom work, and shipping shouldn't be too high if you just get covers and purchase foam locally. Of course, you may have a special fabric choice in mind, and if so, go for it! But if cost and functionality are your main considerations, it's worth checking out.
Best wishes with your bunk restoration project, and Merry Christmas to you and yours!