The Boat Cave got its first real test yesterday. The outside temperature was 24F and the winds were 15-20 mph. It was cold walking out to the barn.
The temperature inside the Cave was 30F - which really was good to see: the insulation is keeping the winds/drafts out. I turned on the Halogen work light, fired up the Big Buddy propane heater and slid an insulation panel into the doorway (still haven't hung the door). Within 20 minutes the inside temperature was up to 40F, and after another 10 minutes the temperature was 50F where it stayed.
Now that's not exactly balmy but I did shed a layer and was able to work comfortably. I only ran the heater on the medium setting so there is more heating capacity. Once the door is properly hung and the gaps around the door are better sealed, I expect to have mid-50s to mid-60s to work in.
As for localized task heat, I can set up shop lights (1, 2 or 3) focused on a work area, shroud the area with black plastic to get 65-70F - plenty for curing epoxy. Here is the bow section with a single light generating 65F.
One issue with the propane heater is that on the medium setting, two 1-lb canisters are good for about 4 hours or so. I may decide to get a larger tank and connector hose. Other than that I am satisfied that the Boat Cave will let me work later into the winter and earlier next spring.