I do not know exactly whats the internal diameter is but I can safely assure you that AW's 18650 (which I assume to be the widest in the market) fits nicely with room to spare (not much but it doesn't rattle). I've got both the M-C 1x18650 and the 2x18650.Don't know the ID of Leef's bodies, but yes, you need >18mm as many Protected 18650s are in the 18.5mm to 18.9mm range. Mine are about 18.7mm.
I do not know exactly whats the internal diameter is but I can safely assure you that AW's 18650 (which I assume to be the widest in the market) fits nicely with room to spare (not much but it doesn't rattle). I've got both the M-C 1x18650 and the 2x18650.