On the S.E.X., I think you'll find that it is the plate chokes, which mount on the underside, which will dictate the minimum depth for the base. At least I don't recall anything hanging any lower, C4S included. IIRC, the plate chokes are just shy of 1-3/4" deep, so I believe a couple of inches of depth will do as a bare minimum.
Of course, this assumes a stock build; swap out a bunch of caps for gigantic film types, and all bets are off. Same for Magnequest plate chokes---I have no idea how far they may hang down.