(note edited 7/22/2012 as the old photo site was shutdown - same content, new photo source)
I built this from parts found around my house. Had an old fan, hard drive for the magnets, and a power supply. Was stumped with what I could use as a base - almost went pure minimalist with just attaching fan to a piece of plexiglas - and then I saw the linksys in the electronic junk pile (yeah, I'm a full time geek and part time hoarder.)
The concept/basic instructions for this is well documented at: http://www.byo.com/stories/projects-and-equipment/article/indices/20-build-it-yourself/401-build-your-own-stir-plate
I pulled the router apart, removed any diodes/chips off the circuit board that were in the way, glued the fan on the board, and attached (just wrapped, did not even solder) the fan leads to the power adapter port inside the router. (I should of taken a picture of that, but I forgot, and I'm not pulling it apart now
). Trust me, it will be obvious, and you can use a volt meter to confirm the right points of contact.
I cut out out the top of the router, where the air vent is to allow clearance for the fan/magnet.
Its not perfectly flat, but the flask does sit pretty stabile on it.
I've used it for a few starters now and it has worked fine - and the price was right.