update to this post

I managed to strap on a fan on the back of my deck, together with a "funnel" to guide the air into the back vent. i used electrical tape to minimise air leaking and connected the fan through a usb c connector to the deck. I used usb hub or else deck didn't wanna provide any power to it.

I'll try to describe how i positioned the fan. funnel was above the vent with opening on the right side. fan was attached to it, sitting in the middle of a backplate.

Don't know the max rpm of the fan but it was pretty loud. so i guess ~5000?

Results were... there. I got ~1 °C reduction for both cpu and gpu. it needed some time to cool the insides but i could repeat the results.

tape sealed pretty good so i can't say i lost much cool air that way. what probably hampered the fan from better cooling is its sideways mount - not directly blowing over the components. mesh and tape over the metal shield weren't helpful.

i'll try to change the design of a funnel; make it narrower so more air goes through the intake.

instead of tape i could put a thin coat of black paint, which should help dissipate the heat.

considering that air is pushed in sideways, i'm thinking of making some sort of internal foam "wall" to help guide the airflow over the motherboard better.

another thing i'm having on my mind is mounting two (or maybe even three) small blower fans right where the intake hole is. i found 20x20mm blower fan that is 6mm thick. if i find thinner, protruding from the deck should be minimal and i bet they'd cool the motherboard better than outside fan. i'm getting custom backplate for that so i can experiment to my hearts content.

if that won't yield at least 5 °C, my next plan is to change internal cooling by adding another heatsink and heatpipe.

fan would have to be 40x40mm to acommodate bigger heatsink. i'd get longer heatpipe. one end would go up and to the heatsink, like how it is on the original. another end would be bent by 180° around the heatpipe holder thingy so it's parallel to the first heatpipe and can be soldered to the new heatsink