Q20 generally fits and made for 3 series and with TB-30. I have the Arca Swiss Z1 (equivalent of Q20) on TB-21 off GT2541. there is a slight overhang but i haven't seen any performance issues. I think the top diameter is same for both TB-20 and TB-21 which is 2.48. Q20's base diameter is 2.68 and hence you will have a slight overhang but I don't think that should be a problem. I am no expert on these, just my 2cents.