Finishing my car in a container....

[The V8 Files]

Being watertight is exactly the problem, water cant get in and moisture cant get out, if your spending a few hours in there then just by breathing you will be creating a moist environment which could condensate down the cold walls resulting in a damp sealed box.

I didn't mean it as a major risk but it might be beneficial to add a couple of vents in it if possible.

Also investigate the paint Bob mentioned as that could help.

Bruce