I would just make a passive air intake and outlet, and if it get’s hot in there.
The hot air will go out the top places vent and natural draw in cool air at the bottom inlet, so I would just place a vent at the bottom and the top of the box.
For the reservoir, if you ask me there is only one solution when your pump is external.
Drill the reservoir and put a drain fitting in like this.
Anything else is just asking for disaster, the day the other thing fails it could kill your crop.
Other option, would be build a “siphon” from PVC pipe. Going from the bottom of the reservoir, over the edge and down to the pump.
That would be easy to do, if drilling isn’t an option.
And I run submergeable pumps now days, for Koi ponds.