MPPT 30-40A and bigger all have a huge (and ugly) heat sink. The whole gizmo was rather unsightly (different brand, not Renogy), and I hesitated a little before mounting it inside. Especially since the most convenient location - closest to batteries - was in the bedroom. I didn't want it in the cargo bay as I didn't purchase a remote display with control buttons - it's all on the front panel - and it would be difficult to see the on-board display in cargo bay. Besides, my cargo bay is not too big and I have other items to store there. Besides, there is more dust, humidity and salt air in cargo bay.
I don't notice much heat at 20A - and it rarely shoots higher because my battery bank is big, compared to my energy needs, so in the morning it starts from 80-85% Full and by the time there is enough sun to generate 30-40A, my batteries are in Float and draw mere few amps.
It makes soft clicking sound when switching between the modes (not very often), and the sight of a big white brick on the bedroom wall is something to get used to. Thanks God the display is gray LCD (backlight when needed), not a bright red like Trimetric or some controllers brands. 1/4" wall panel can be a challenge to mound 5-6 pound brick.
My manual recommends 6" clearance above and below. Check your manual and go ahead.