I'm just getting into home projects, but after doing a few I decided breathing flux fumes probably wasn't too good for me or my kids. I could just buy one (prices range from ~$40 to well over $100), but decided to build my own. It ran a bit more than it probably needed too - I probably could have shaved a few bucks on some of the parts, but overall I probably came out a little ahead, and learned a bit in the process.

Parts list:
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

Corner holes were marked by holding the fan on the outside and then using a very thin round file to mark the plastic.

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.

Step 2: Attach the Parts

All downhill from here. Turned out that the mounting holes for the fan were not exactly aligned, so I had to use the deburring tool on one of the top holes to get the bolts in.

Switch and jack each had a removable collar to secure them. I soldered extension wires to the jack and switch before mounting them so that I'd have an easier time making the necessary connections. Final wiring was tested by just twisting wires together before doing the final soldering.

I just used black copper wiring to attach the filter because I had a big spool of it. Originally I had planned to do a fancier bracket on the inside, but the realized the wire would block less of the airflow.