You can buy a hood for your 15 gallon for fairly cheap. I use these: http://www.fosterandsmithaquatics.co...81&pcatid=3781
Some people also simply use plastic wrap to cover their tanks--but you do need some sort of cover in order to keep the temperature and humidity level of the air above the water high. If the air is cold and dry, the fry may die when they go up to take their first breaths.
Excuse me if I seem rude or presumptuous, but breeding is very expensive and very time consuming--it can also be very disappointing. Generally when people first post about being interested in breeding my reaction is that of discouragement. After all, there are thousands of perfectly beautiful bettas dying in cups in the pet store. Why risk the health of the ones you have by breeding? Unless you have high quality fish from a breeder, whose genetic history you know, and a clear goal that you plan to achieve I think you should devote your time and energy to the fish you already have. Again, I don't mean to be rude, but far too many people get into breeding bettas without knowing what they are getting into or why they are doing it.