Since my last post, I finalized the decision to go with a 38mm mount. I cut three rings with my Rotozip, each of which was imperfect. The first one, despite 2 measurements, was too small. :expletive deleted: Luckily, I can use it inside the cone's shoulder. The next one was a different diameter at the start and end of the cut :eek: Luckily, it is close and I can shim the short segment. I think the center peg was sliding out of the pivot hole due to the surface I was working on. The third was better but still not perfect.
The Rotozip won't do a 38mm hole, I don't have a approproate hole saw, and I couldn't find my circle cutter. So, I punted. I cut a hole larger than I needed and attached 38mm-3" centering rings. This method isn't what would be considered good modeling, but it works! I have yet to drill the holes for the 24mm canted mounts or notch the sides for the fin extensions. I decide I'd mount the added fins on bars that slide up through the centering rings adjacent to the body. I'll hive to look around for some clear material and use Lexan fins. If I can't find cheap clear plastic bars (nominally square tubes), I may just go for wood fins.
I notice sandman from Excelsior has a decal set for a BT-101-sized MLAS model. I assume he can scale them so, if mine survives its 1st flight, I may ask if he can make be a set.