You would just need to be able to heat the small space that your snake is in, which is why I suggested bags+tub - the bags to keep your critters insulated from the heat source, the tub because it's a small space to heat, and you can put several bagged critters into one tub if needed, and you can insulate the tub by wrapping it in blankets or towels.
I suggested using a tub because most herpers have spares laying around anyway, but if you don't have a spare then something like a Styrofoam beer cooler would work and is inexpensive. Just make sure you secure the top in case a snake manages to get out of a bag.
Whichever you use, you can take a digital thermometer and put the probe in the container and set it up so you can monitor the temp without opening it up.