I got a Uctronics 19" 1U Raspberry Pi Rack Mount (https://thepihut.com/products/uctronics-19-1u-raspberry-pi-rack-mount-with-ssd-mounting-brackets-holds-5x-rpi?variant=42461815406787) but I wanted to add fans, with a fan controller board, so as I couldnt find one, I made my own - these fit the sleds, have space for heatsinks and also fit the controller boards (https://thepihut.com/products/fan-controller-for-raspberry-pi?variant=39578362577091)

You need 4 M3x12 bolts to fix the fan, and the holder needs 3x M2.5x16 bolts. This will either fit the sleds for the rackmount here, or I have found they will also fit other mounts - I do however intend to make my own “mini rack” in future - watch this space!

This will work for both the pi 3B and 4B variants, and possibly others too as they all share a mounting footprint. (There is a "Tall" version as well for taller heatsinks- but uses the same M2.5x16 bolts)

Designed to be printed at 0.2mm - no supports, 15% Infill - use PETG due to warm environment of the Pi