I used a computer 4" muffin fan.
Cost was about $6.00 and current draw less than 1 amp. and available in 12 VDC.
They are only 1 in. thick and run on ball bearings as well.
I adapted mine to fit an old rear under the seat school bus heater and am more than satisfied with the results.
As to a rheostat, forget about it, just flip the switch off when it gets too hot!
That wont happen often as your biggest heat loss will be the floor area with slits for peddals etc and (if like mine) the punched out holes for drainage and anti slip.
Because the coldest area is the floor, I mounted my heater upper right front corner of my cab as my head is never there, blows heat down to where it is most needed and since warm air rises the upper body area is always comfy.
Also up front gets the blown air onto the windshield. (you can slightly tilt the heater forwards to assure that or add a small metal deflector to accomplish that function)
Good luck, have fun!
