If youre going with happ's parts(supers, ultimates, pushbuttons, etc) most of the description pages on their site have either mounting diagrams or dimensions
My guess if youre building a box type enclosure would be about 3" below the sticks. FIgure your sticks will be about 1/2 way from the front edge to the back, so if its slanted, maybe 2" at the front and 4 or 5" at the back, depending on how deep it is. THat should give a nice slope too
For the record, Ive got the older style T-sticks from ultimarc, bottom mounted. They ould need about 2-1/2" clearance to not rub on the bottom. Horizontal microswitch buttons are about 1-1/2" clearance, when mounted thru 3/4" in panel