Adding enclosed heated underbelly. Can it be done?

OP here,
I thank you all for the ideas and input, even the ones that varied a little. All these things are a big help. It's nice to be able to glean info from others that have done it.
I'm going to climb under and thoroughly evaluate a plan of action and do something with mine. My floor gets really cold in the middle of January. But I'm not going there until it warms up in the spring.