so about 8" (200mm) plus a hole in the shelf above and another 1 1/2 " 30mm is plentyI like to leave 2" if there's room, 1" minimum (with limited hours constant use ) or I'll take my TT off the top shelf and go analogue only.
A pierced cage ( as opposed to a wire one) that acts like a heatsink can help diffuse the heat over a wider area but you still need some ventilation (so ,1" min but less concern about leaving it on for longer).
Just an uneducated approach mind you.