Id suggest hopping on ebay or amazon and finding a variable voltage controller for under $20
Generally they are 3-12v controllable (quite the variety of ranges), and they would be fairly easy to do a DIY setup.
The easiest method would be to find a sata to molex power adapter to maintain plug and play with sata, and chop the molex end off to provide the 12v to the voltage controller.
Then find an old fan power adapter to hook the fan up.
You are stuck using a knob or screwdriver to adjust voltage, but they generally have upwards of 10amp control.
Find one with a heatsink on it, that is a good sign it can handle lots of power.
I beleive they were titled as "dc to dc variable" in some listings.
here is an example of a SMALL version that is rated for 2amps constantly, but burst to 3 amps. This is a 5 pack, so not exactly what you want given the price.
Just a reminder, most fan ratings on their stickers are for max rpm constant use. Very few of them show the initial spin up power draw, which can be as high as 5x the constant rating.
This means you may want to have a small capacitor added to the power input side to help buffer the initial surge spike.