Here is a case I designed to fit a buck converter for the ender 3. It uses this converter off of amazon.com:
It makes use of the existing feed to the fan, and only requires a 6" jumper (red, black) to go from the melzi hotend fan out to the converter input.
This converts the ender 3's 24VDC fan voltage down to 12VDC so you can use the efficient & quiet noctua fans.
The lid clicks down on the case, and it is a perfect fit sideways between the melzi and the frame. No screws required!
1.) Remove cover
2.) Disconnect 12VDC Red/Black from rear two screw terminals (turn left a lot or they will be hard to place at this angle later on)
3.) Connect red/black to +/- OUT on buck converter
4.) Connect jumper to red/black on Melzi (the angle sucks, but it can be done without lifting the board)
5.) Connect jumper to +/- IN of buck converter
6.) Power up machine, make sure hotend fan comes on
7.) Put cover on case, and tuck behind board
8.) Put everything back together
Total commit time: ~15 minutes