simple 12 volt heater
You could just use a 12V automotive heater like this on amazon (but really any store with auto parts should have them):



The benefits are:


It's a tested heater that should not burn down your building (unlike a true DIY using some type of "hot box" that might even invalidate your home insurance)


Don't forget to keep flammable material well away from it or any heater / light bulbs

It's designed to use 12V from automotive-like batteries, that are often used with solar panels, so all you need is a 12V socket and it's "plug and play"


I'd recommend trying one out in person, I tried one once that was barely warm and you couldn't even feel the air a foot away, while others are almost like a hair dryer (but designed to run safely longer than a hair dryer)