Now that the jewel-like A10-0T mini engine is back in production, here's a booster suitable for them. Now, given the total impulse is sub-2.5Ns, you shouldn't think about sticking this under a heavy sustainer but a suitably light built BT-60 upper stage powered by an A8-5 would be tons of multi-stage fun with a much reduced risk of loss because 2 x A motor does not equal an orbital insertion attempt ;-)
And, the booster has the cosmetic appearance of a whacking HUGE engine bell adding a certain amount of pad presence to an otherwise average looking 3FNC design. Go ahead, knock yourself out!
Fits the Semroc Centurian, Vega and Goliath (built with a single 18mm motor) and other rockets with a similar BT.