For something close to your interior light/festoon proposal, how about add-on van interior lights? (link goes to a cheap set picked at random)

Runs on 12V, reasonably sealed against at least dust ingress, usable in 1-to-n module lengths depending on your needs - which seem to run more towards flood than throw, given what you were initially considering.

May or may not meet any requirement for output, though - we'd consider 30+ Watts, and 2,000 to 10,000 lumens 'high powered' round these parts, but maybe a cheapy COB or zoomy light with its switch frankensteined to a 12-3v buck converter would work.