Ultimately I decided to go with an Astrosystems spider and holder. Tip to tail it is right at 9 inches. My 5 ply drum shell is 10 inches tall. I'll be making the top and bottom rings out of 1/2" thick plywood. The rings will be 1.75" wide with a groove inset from the inside edge 1/4" for the drum shell to fit down inside. I'm also using 1" aluminum tubes to fasten the two rings to each other (trapping the drum shell between) and supporting the spider. The completed UTA should end up being right at 10" tall, not counting the Aurora Precision brackets mounted to the bottom of the lower ring.
The UTA will end up being 13 1/8" inside diameter on a 12.5" scope.
I could make the UTA lighter by using Kydex or similar, but then I'd have to use thicker plywood, so I think that's a good trade for much more stiffness.
The focuser board, while 1/2" thick on the side edges will be radiused on the inside to match the drum shell. This removes some weight from the board and moves the focuser closer to the secondary. Move it in far enough and I may end up using a 2.6" secondary instead of a 3.1". which translates to a bit less weight and (more importantly) less leverage on the spider at lower altitudes.
Careful, or the focuser drawtube will stick inside the UTA I.D., which causes two additional diffraction spikes in the star images.
If you use a 13.125" I.D. on the UTA and any normal focuser height, a 2.6" secondary is more than large enough.
My own 12.5" has a 14.5"I.D. on the UTA and an un-radiused 1/2" focuser board and a 10.75" secondary-to-focal plane distance, yet a 2.6" secondary is fine.
And not a bit undersized, as 2.45" is adequate to provide a 70% illumination at the edge of my lowest power 30mm eyepiece.
BTW, simple math shows that if you don't want the UTA I.D. to vignette the field in any way, it's minimum I.D. should be the
primary mirror diameter PLUS the field stop diameter of your lowest power eyepiece. My UTA was a tad oversized, but the focuser drawtube never intrudes, even with a Paracorr 2 in the focuser.
If the low power eyepiece is a 31mm Nagler, the minimum UTA I.D. is 14.15" If the low power eyepiece is a 30mm APM UFF, it can be 13.93"
But even if the scope is designed to only use 1.25" eyepieces, it should be about 13.57".
13.125" will vignette just about all eyepieces longer than 12mm.