I initially put those basic interlocking corners on the OT:

but because the T39 is a couple of inches deeper than the OT12, once the front ones are interlocked, the rear ones can't really be aligned properly.
I then replaced the front top ones on the T39 and the front bottom ones on the OT with those ones:

and it's allowing me to adjust a bit better the alignment until the rear ones are in place.
Ideally I'd have to fit both cabs with the later ones, but I only have one set of those and the hole patterns aren't the same, which means some screw holes to fill ... For consistency in the look (for what it matters), it would also be good to not have to mix different types of corners on a cab.
It's no big deal, but I'd be open to suggestions, or happy to find out what others have used when using this combo as a bass rig (the only legit case of stacking a top on a sub).