My, my; the last thing I expected was to toss a hand grenade into the fuel dump with this question! Some great discussions above; it surprises me to find it still going on two days after I posted the first query.

I'd already decided that, in spite of Joe's use of the external technique in developing the Spring Run, the ability to use the internal strongback and forms to keep everything true over unexpected and ill-timed delays in the project was important enough to warrant the conversion in my own case. The one thing I can confidently predict is that unpredictable delays will happen, and keep me away from working on this it just when it's least helpful. Totally serendipitously, when I went into the lumber store to buy the plywood today, sitting there was a nice piece of OSB-lauan sandwich 1 3/4 thick, reasonably rigid and completely free of warp (unlike the 2x4's I can find these days!). Strongback now solved, with appropriate multiple-point support and stabilization planned.
My thanks to the multiple contributors of great ideas above! It's what's made the board so fascinating to follow as I've been planning the project. Thanks also to Nick for starting and supporting the board; it's often said but can't be said enough.

Craig

: With the hull exterior glassed there is no need anymore for

: With just the exterior glassed the hull can still change shape. Many on this
: board have written about their experiences with this. Therefore, I like
: the idea of being able to put the strongback back in until I'm ready to
: glass the interior.

: Tom