best bet is just a basic thermal pad made for this purpose like this. Foam board needs to be covered by a plywood subfloor which is obviously not what you want. Any solution here isn't going to be too efficient.
Popular alternatives to
Non-Slip Thermal Insulation...
De'Longhi Comfort Temp Full Room Radiant Thermostat, 3 Heat Settings, Energy Saving, Safety Features, Nice for Home with Pets/Kids, 27" x 6.5" x 15.5", Light Gray