I've made a few bamboo backed osage and ipe bows and a few trilams and didn't use a hot box to cure the Smooth On and they're all fine. I plan on tackling a glass bow in the near future and don't plan on using one.
To add to what Mr. Holm said, if you wait until things warm up, on a nice sunny day, you can use your vehicle as a hot box if the form fits in it.