wiring in a thermal resistor shouldn't be much of a problem..Will get you a variable speed fan (by voltage drop)
Details above my paygrade..
Probably easier ways..
https://electronics.stackexchange.co...controlled-fan
More info:
http://projectschematic.blogspot.com...and-tl082.html
simple snap switches (like on a furnace) will at least get you on/off
Should be some Chinese parts out there ..
would need 4 wire computer fans..
https://www.ebay.com/itm/12V-PC-CPU-...4383.l4275.c10