Here's another idea. I recently needed to build a travelling exhibit for an art exhibition and decided on buying some hollow core mahogany doors. They cost $17 each and are 36" wide by however high the average interior door is. I hinged mine together because I needed to disassemble for transport, but you could take such doors and semi- permanently connect them with firring strips or moulding. The corners will make the structure self-supporting. The surfaces can be painted a neutral gray (paint the area around the enlarger black). Some Duvetine can be purchased through Matthews Studio Equipment (Burbank) and used to create a light trap at the top. Just staple to the joists and trim to an appropriate length. You might even be able to attach some shelf standards to the solid ends of the doors and give yourself the storage space for chemicals, paper, glassware etc. that you'll no doubt need. This system might give you the rigidity of walls without the expense and trouble of working with studs and drywall. Let us know how you make out with this.