I've done the same thing as Oliver but without the ratchet straps. I just use 3/4" fiberglass packing tape and I'm usually able to slip a putty knife in, wedge it out a bit and pull down on the tape and lock the deck in position. Make sure you wash the deck and hull of blush so the tape sticks well. Don't wait with putting some spreaders in the boat. Do it now because the epoxy is still a bit flexible and hardens more and more but actually takes a few weeks to get stone hard. You didn't say how much it pulled in. I had one boat years back that really collapsed on me (my fault or not keeping forms in it) and I had to open it up about a half inch. I used the tape method with the glue gun like in the manual but put spreaders in it and wet out the tape in sections in front and behind the spreaders through the hatch openings and cockpit. I let it harden for many days then removed the spreaders and wet those remaining areas out.

Also, if you have a slight taper on the hull and deck edges, the epoxy will run into that space and weld it together beyond the strength of the glass tape. See if you can use the forms and open it up a bit for a few days.