Of course, while I was working on it, the only 13mm booster available became decertified 

But I can still fly it.  And quite frankly, no matter what the sustainer motor is, a full A10-0T in the booster is likely to result in lost rocket parts. Fortunately, I have a small stash of really old 1/2A3-0T boosters which greatly improve the chances of recovery. So, if I gotta use a decertified booster, I might as well use one that's better suited to this itty-bitty rocket.

I made one change from the plans.  I moved the booster fins forward.  This helps destabilize the booster and makes it tumble after staging, so it doesn't come in ballistic and wreck itself.