We use the old porcelain coated bathroom heaters in our box stands. Pick them up at flea markets and yard sales. We clean out the orifices with a torch tip clean-out bit and blow them out with compressed air. Quiet little blue flame and if you can find one with a flat top it'll heat one of those little tin-foil trays of cinnamon rolls very nicely. That'll buy you an extra hour in the stand at least on a cold morning.
edited to add: they are getting difficult to find, though. And I apologize, you said portable. We plumb ours to LP tanks under the stand with rubber gas-line and quick-connects.
Edited by Slow Drifter (08/24/15 05:08 PM)
"I have no idea what WW-III will be fought with, but WW-IV will be fought with sticks and stones."