Inside the box would be interesting.
But it’s probably at least 40C as the Odroid has an active cooler and i wouldn’t expect more than 20C differential from the CPU core to the air around it.
My suspicion holds: The airspy is just getting too hot to use it at 20 MHz reliably.
Stick some sticky tape heatsinks on it or something.
I’m using a computer case 12 V fan running on 5 V.
It runs nice and slow but reliably.
I guess the enclosure is quite bit with all that gear in there?
No matter my recommendation stands, build a non rain proof well ventilated “shade box” for the actual enclosure.
Much better than insulation as the heat generated in the box is kept inside by the insulation.
With the insulation in place you would need a fan ventilating the enclosure to the outside, but that’s problematic due to water ingress.
But i guess if you stick on 30 cm of duct below the fan where the fan sucks in the air, it shouldn’t be a problem.
The fan across the Airspy i’d still recommend even after reducing the temperature in the box.