I had the same problem. It wasn't fun.

Figure out the proper width where each form used to be. Cut a stick and put it in - dab a little epoxy on the end to keep it from sproinging out (you might have to epoxy a little block to wedge it under (just chisel off the block later)). Put lots of sticks so the lines are smooth and don't wave in and out at each stick.

Now put the deck on and tape the ends - don't be shy with the tape. Throw a loose loop of rope around where each of the sticks is and twist a short stick in the rope to tighten it up - by slowly and evenly tightening all the ropes you can pull it back into shape. Once it is in shape tape the heck out of it, then tape and epoxy the inside seam.