We always make not useful suggestions because we don't know exactly what the plan is.
Yamaha makes another mouthpipe attaching appliance for the marching 201M convertible tubas which I consider to be much more robust and something designed to be taken apart and put back together many many times. It's a simpler design that uses a tension screw, and a regular slide insertion system.
Of course, the bore of the BB-201 only achieves .728" i.d., but it's such a simple design that it can be duplicated pretty easily with just a little bit of silver braising, drilling, and thread tapping.
Again, all of these comments are likely non sequitur, because we don't know what the plans are in regards to the original inquiry.
Just as a general comment that doesn't fit into this inquiry though, the factory BB-201M .728" appliance works quite well with the popular 45SL-P mouthpipe (and shop-made copies of it), because that mouthpipe usually does ~not~ achieve 19mm prior to the first piston - and thus that Yamaha slide and tension screw appliance is just about ideal for for those who prefer to install the 45SL-P mouthpipe on a tuba with the ability to remove it.
For those who prefer to buy things (versus making things), the Yamaha model BB-104 mouthpipe receiver attaching device (whereby it attaches the receiver to the bell - but it's also removable) can prove to be handy. Both of these items are pretty helpful for early-stage frankentubists who don't yet have a lot of fabrication skills.
Getting back to the specific need here, that alternate Yamaha system (oem installation) relies on a combination of compression fitting and cement. I believe if Yamaha completely trusted the compression fitting, they wouldn't put any cement on there, this is why I would be tempted to go over to a slide and tension screw system, basically as is seen on a sousaphone neck or other detachable mouthpipes made by other manufacturers (B&S, etc.) as I described the 201 marching instrument.
Being redundant, If the bore is somewhere in the low .8XX" range, I'm pretty sure I'd be tempted to just fabricate a short slide and tension screw system from scratch with some inside/outside tubing of that bore size (or tubing of very close to that bore size) and call it done. I really believe that such a system is much more robust as far as being taken apart and put back together numerous times versus the compression fitting and glue system. Stating the obvious here, pre-drilling a piece of quarter inch rod or buying hollow brass 1/4" w/1/8" hole tubing, perhaps shaping down the ends to make it look a little more stylish, brazing it on to the outside tubing to be used, cutting it with a little saw, and then tapping threads into one side of it would provide the finished product with neither very much storm nor stress.
