I used to have a 7 x 14 enclosed trailer for construction. It was not insulated and when I needed to heat it I used a milk house heater. It was $10.00 at lowes on clearance amd worked great. Kept the doors closed and the unit on low I could work in a t shirt in January .
I'd ditto the RV heater.. Outside vented, small, put out a good amount of heat, thermostat controlled, runs propane, 12VDC or 120VAC. Duct it as necessary / convenient. Probably cheap if not free if you know someone who has an old camper.
A forum community dedicated to professional woodworkers and enthusiasts. Come join the discussion about shop safety, wood, carpentry, lumber, finishing, tools, machinery, woodworking related topics, styles, scales, reviews, accessories, classifieds, and more!