I had the same problem which I alleviated (made easier but still not as easy as it would have been if I had prevented the hull from closing in the first place) propping the hull open with scrap sticks.

- Start by hot-gluing 1 inch long strip cutoffs about an inch down from the sheer line every foot or so on both sides of the hull

- this gives a place for the props to 'grip'.

- Use sticks to prop open the hull wedging them under the blocks you hot glued on.

- Glass and epoxy the outside joint.

- Reach inside and knock out the sticks and blocks (harder than it sounds)

- Glass and epoxy the inside joint.