Mercury Vapour exposure unit:

[Switch]

great, sounds spot on for you.

[Andymac]

That's a thing of beauty. But yeah, vent it, just cut a few slots in the ply near the top.

Judging by how you made this - the fluorescent viewing lights on the insert over the MH is really nice - I think you'll figure out the top, either compression with some foam or get some rubber and a vac. You can get support struts at a hardware store, or bust a gas shock off a wrecked car with a rear hatchback.