Here's my version, front & back:
About the only mods I made from the UofM design were to put the filter on the back of the fan, and to seal it down all around with duct tape. It sucks down against the fan better that way, so instead of trying to blow the filter off it's being pulled into a stronger configuration and the tape job doesn't have to be as robust. The filter is a 3M Filtrete 1900 Maximum Allergen model, about $20 at Lowe's. Be sure to get the flow direction right when you tape it on. There are arrows on the filter's edge and it's not hard to figure out. The fan is an old $10 cheapie. Luckily 20"x20" seems to be a common size for both box fans and filters, and everything meshed right up.
Cut to the chase: Yes, it works. I haven't had a sniffle in over 48 hours now. While I was at it, I got one for the central air system too and effectively turned the whole house into one big allergen filter.
But I still popped for a nice $130 ready-made HEPA purifier for the office. Quieter, and with nice timer and speed controls. It seems to work about the same otherwise. Effective, cheap, quiet: pick any two.
Damnit, wish I'd known this two years ago.