Portable heaters.

We did well in uninsulated buildings with box store oil filled radiator style electric heaters - except where it was a cathedral cieling (two stories open).
However they are very slow - turn them on at least an hour or two before you need them, and the floor or in your case ground will be very slow in responding.

True radiant heaters work best when there is direct line of sight from heat source to objects/people you want heated. Great when you really don't want to heat the air and/or have large space.