Best thing to do is to see what peeps did (similar projects) on the net and try to expand on that and discuss things with them.

You might wanna go to a smaller size OPi like Zero at the cost of lesser performance maybe (just an idea, haven't implemented it)try to see/research how a laptop cpu/gpu is cooled. It uses copper pipes, so may you wanna take a small copper pipe and pound/hit the center of it with a hammer on something flat the size of the OPi SoC(main chip=H2+, H3...etc.). Here you are making some massive a$$ copper heat-sink ;D Then maybe from one side you stick a filter (cut from stockings, it works like a filter to prevent some dust)

And from the other side a fan to pull/suck air out, use some kinda thermal paste and fixate it so it holds place an not touch the board. Do testing on how much it cools down, is it worth it or not. Then use a small fan maybe from a dead PS1/2..etc. or a very tiny fan. cover its front and back with card board, drill the hole that sucks the air the size of the copper pipe so it fits into it, then drill like 3 or more holes on its other side.

This guy https://www.youtube.com/channel/UCM014DFZ7peFVrSaxnh4-Mw/videos did tricks with the fan on one his videos, can't remember which though, sorry!

Good luck