I liked this idea a lot. I have a 30 x 30 outbuilding and keeping the whole building warm is pricy. I did what Lee suggested. Two 4 x 8 panels of insulating board from Lowe's, around $25.
Went on Amazon and bought a small 500 watt heater and a thermostat to cycle it, $42.80. I wanted to be able to do a scale boat in it so I cut the board in half on the length. Cut each 2' section at 5' on the length and then taped the 5' section and 3' section to create a hinge. Put each side up and you have a 3' x 5' box 2' high. Used the second panel to make a top. Everything folds flat for storage.
Started using it today. Works great.