I've looked into the idea of a heater inside a paint booth some more this morning and here's what I came up with:
https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_id=10657&p_table=STANDARDS 1910.107(j)(4) In particular.
There's a lot of considerations to take into account, and if you're unsure of any of them, don't do it. For example, it needs to be an *infrared heater*. It must be at least 18 inches off the ground. The area must be ventilated for at least 3 full minutes before turning on the heater. Ventilation must be maintained during drying. That document should be read very carefully before attempting installing a heater in a painting booth.
The reasons for all these safety precautions really should be obvious. We're talking about aerosolizing very flammable substances in an enclosed area. You're essentially standing inside a bomb of the fuel-air explosive variety. Adding an ignition source seems foolhardy at best. And here's why:
For the homemade painting booth in some room in your house, it seems to me to be a far better idea to heat the room that the painting booth is in, and make sure that all airflow out of the booth is properly filtered, rather than attempt to heat the painting booth directly.
If people decide to ignore this advice and either blow themselves up or set their houses on fire, my conscience is clean.