Here are instructions for how I put together my own large scale exposure unit. An excerpt from an email to a friend:
"I included links for the electronics, (unfortunately Amazon is still the most convenient supplier for some items). I didn't include the prices for the lumber, and some of the stuff you can find at the hardware store. The side panels I got at the shelf aisle at Home Depot. Keep in mind is the distance from the light tube to the glass: keep that distance to around 5"; I've gotten really good exposures that way.
I use TZ emulsion, and I get really short exposure times with my setup. Usually 40 sec - 1 minute exposures, depending on the image.
The most expensive thing for me was buying the glass. I had to get a custom order, I think I ordered it 3/4 " thick glass, that was shatter proof and without UV protection (cause you definitely want UV rays to go through for your exposures).
You can make you're own smaller one using 24 inch UV light tubes and it will be a lot cheaper.
You can follow these instructions and cut all the sizes in half.