Buy board, remove heatsink from board, open heatsink find carefully (look up youtube vids for technique). Apply new thermal compound to the APU and put the heatsink back on. If you have a PSU with the right header use that, else a metalfish 500W is ok. Buy 3 cheap but good quality fans (arctic pN (where N is the diameter in centimeters) are my choice).

Make a cardboard box with good airflow, you can stiffen it with wheatpaste paper mache if you want to go super cheap, or build a fancy wooden one or whatever. Make it so one fan passes air across the box, one pushes air into the fins ( you can fabricate a cardboard bracket, use hot glue to glue it in place whatever) and one replaces the fucking awful metalfish fan (remove from case and mount new removing old)

Design your box so air flows over the back of the board too, so it should be held off the ground by something. You can screw into cardboard stiffened by wheatpaste, so the case can be well designed and literally made of scrap and painted nicely if you like. No need for a printer, that's just easy if you are bad at arts and crafts.

You'll need to salvage a latching power button or switch from something as a power button. I use a missile switch from some old tool I scrapped years ago lol.

You can go super fancy mounting coolers and stuff for quiet operation but the basics are make sure you have a PSU, new thermal paste, modify the heat sink to work with fans, and a case with good airflow.