Concrete insulated blankets work well, there are even some companies making a skirting system using similar.
If it was my money, I'd use extruded foam board and fashion a holding system to keep them in place. Another measure that appeared to work well was to use heat lamps or electric heaters underneath. A member of this Forum was work-camping in ND using the above system and was able to hold 65f under his trailer in ND Winter climes. He had a temperature sending unit attached to an axle so was able to monitor the temps. The only downside he had was the growth of weeds underneath and was concerned that the growth might come in contact with the heat lamps. We discussed vegetable crop harvest and rotations.:)