I'd spend $100 on an electric oil filled radiator and call it good. I've got a couple of them around here. Leave one on for a couple of hours and it'll turn an 8X12 space into a sauna.
I’m sitting three feet from mine in my 12x16 hunting cabin in Alabama. I put in on the max when I get back from my afternoon hunt. In an hour I have to turn it down to the minimum watts. Last night it got down in the high 20s. I had Morning coffee in a nice toasty cabin. I did a lot of research on heaters when I was buying stuff this cabin. For me this electric oil filled radiant heater seemed to check all the boxes. It has worked out great.
