I did some polyester resin repair work on a C-1 and had the same problem with cold weather. I found an 800 number on the can and was able to talk to a vey helpful chemist. In my case they said adding more hardener/catalyst would not help - getting the temp above 70 deg F. was the only way. If there isn't a phone number on the can, try an internet search for company information. They can probably tell you the minimum curing temp, recoat times, wax additives etc. If not, the plastic tent/ligthbulb idea sounds good to me. - Keep at least 2 feet between the bulb and the boat or tarp to avoid fire hazards or scorch marks.
good luck, Brad