Ok, first this is only money saving if you have salvage parts laying around ...old computer case fans and boxes to mount them. A home-made mag stirrer was fun to build but I ended up using 3 different fans and two power supplies before I was through. Learned some electronics stuff like rheostats suck for this task and I started with a rheostat wired directly to a case fan from the manufacturer ..I did end out using it but it barely delivers any real control of the speed. There are better options for DC Voltage control, but those links are above, and my primary goal was this was just my time and very little money ...magnets cost me < $2.
First mistake was picking the most powerful fan. In the end the best choice was a low-RPM (advertised as "quiet") 120MM PC case fan. Second mistake was choosing the most powerful AC adapter, derrr ...I had never heard of one but it turned out to be and AC/AC adapter My first DC fan ran for a short time on 12V AC [eyeroll] ...reading glasses help when you're my age.
To prep I brushed on a couple coats of water-based poly. Especially the bottom was uncoated plywood and would have soaked up every drop of e-juice. I mounted the fan with blades intact right to the bottom of the box with no clearance. After testing fans with no blades it seemed to run smoother with blades ...that forced cushion of air seemed to be stabilizing.
with the fan spinning I took a Sharpie and made a dot in the center. I glued on a large thick white nylon washer, using the dot to center. I tested it for rotational balance, adjusted and after satisfied I set it in the sun to dry. then I did the same with a large thick metal washer.
The magnets were a little tricky. "Opposites attract" so the two flat sides which snapped together each became the tops. They worked better being at the outermost perimeter of the metal washer. Each magnet was rated at a 2.6 pound pull but were relatively small and cheap (.81 cents each) I ordered and picked up at a local Grainger
Turned out OK. I may still buy a real one but this turned out to not only work, but fits perfectly in my new mobile Lab box ..check it out. Here's the bottom. I drilled four holes at the corners and mounted heavy rubber feet after coating with poly (the raw wood)
In the end it was simply a low-speed 120mm case fan with some magnets glued to washers, wired with a PC-type rheostat (salvaged Thermaltake). There's no power switch just a DC adapter jack punched through the back ...if it's plugged in it's hot, but if the rheostat is at 0-75% it's not spinning.
Best part is size. It's low profile made it a perfect addition to my new mobile lab box (another thread)