The ceiling is the problem, I do have large sheets of black plastic that we have used in the past in ceilings for this purpose, but for this permanent darkroom a monster skylight is above the darkroom space so hanging a ceiling is problematic.
We will build a roof structure above the 8ft drywall , use the plastic, and hopefully use some sort of lightweight blackout material for added protection.
This roof will hopefully a lightweight solid attachment that effectively blocks out light from above.
Air, Exhaust will be supplied from the roof into specific areas of the lab.
A properly framed ceiling structure of wood will hold plywood panel and sheetrock (theres a thinner version made for ceilings). If well made, a plywood ceiling/roof can be used as a storage deck for extra gear, look at the way decks are made.