Here's a new technique that solved the problem (noted on the journals on Nick's site) of matching up the hull and deck of his S&G design.

I stitched up the hull around his cross sectional forms, got a snug fit, and put in a small fillet between the forms. I then stitched the deck pieces together, dropped them on top of the forms (with the hull still on the forms), tightened the stitches, and duct taped as needed to get all the joints tight (including the sheer). I then applied a bead of thickened epoxy to outside of the deck joints. This did not add much strength, but added enough rigidity (along with the stitches) to allow me to pop the deck off the forms and tape the seams with no movement. Once all the seams were taped, the two halves fit together easily.

It probably helped that I used 1/4" plywood - that left more of a seam on the outsie for my epoxy to get into.