Portable heaters.

The kerosene heater worked for me. This fuel burns pretty clean requiring a minimum of fresh air to keep the headaches away.
If you burn diesel in it, it needs a lot more fresh air.

You can also get the kind that screws onto your propane BBQ tank. A pair of those will warm it up in a hurry. Plus they have the radiant reflectors on top. Just one will blow the budget tho....