The Nike Ajax goes pretty good on three D12s. However, when I build my other kit.I think I'm gonna use a 29mm MMT. I wasn't a big fan of the centering rings provided in the kit. They seemed awfully flimsy.Also the interstage area (where the 1st and 2nd stages attach)
needed a LOT of reinforcement, there wasn't much support for such a long upper stage tube.
My 2 cents
Mark T