: without the strong back what keeps the forms from sliding to the center of
: the kayak?

Stubborness. And a few drips from the glue and epoxy.

Your problem is going lie with removing them, not with keeping them in place.

When you assemble your forms you should cover the edges with a sacrifical layer of masking tape (or duct tape). The glue which oozes from the seams between each strip does not stick very well to masking tape, but it does stick a little.
A sharp tap with a hammer on the edge of each form will either break the glue bond there, or cause the tape to pull free from the edge of the form. Either way the form eventually comes out, and at worst you'll have to sand off the drips and the scraps of tape that remain.

PGJ