Project Box (8" x 6" x 3"); Radio Shack 270-1809 $6.99
DPDT Rocker Switch; Radio Shack 275-695 $3.99
Fan, 12VDC, 99CFM; Jameco 1585389 $11.95
Weller Fume Extractor Filters (3 pk); Jameco 684828 $7.15
Jack, DC power, Male 2.1mm; Jameco 151590 $1.19
12V Power Supply; Jameco 252823 $13.15
wire, nuts & bolts, solder, etc. I had lying around
total $44.42 (with two spare filters as well)
It works well, isn't too loud, and now I feel better about soldering.
Step 1: Prep the project box
I used a hand held drill with wood bits - seemed to do fine on the soft plastic. You can see that I roughed out where the center of the fan was because I didn't need holes there, although I clearly wasn't always too careful about where I drilled. In retrospect I think I should have cut a single large hole then used a wire blade-guard. That would have provided less resistance in the outlet. I used a deburring tool to clean up the edges. The switch and jack holes were made with larger drill bits - used a caliper to figure out the diameter of the shaft of each.
For the front hole I used one of the filters to pencil in the size, and then measures out a slightly smaller hole. I used a Dremel tool with a cutting wheel. It left a sloppy edge that I cleaned up with a razor-cutter and then a sanding drum on the Dremel. I used a punch to mark the holes for the wires pretty much by eyeball, and then used the wood bits again.