I use a direct fired HLT but I have read that some folks use bucket heaters as portable heating for igloo HLT's. It seems they would do a good job of heating 5 gallons or so to strike temps. The other advantage is that you could set them up with a temp controller and timer to turn at say 6AM to start heating your strike water, then the temp controller would keep it there until you finish your coffee and are ready to start brewing.
Something is always fermenting....
"It's Bahl Hornin'"
Kegged: Sour Saison, Pale Ale, Aggie Ale, Firestone DBA, De Koninck Blonde
Bottled: Belgian Quad (Grand Reserve), Derangement (Belgian Dark Strong)
On Deck: Pliny the Younger