How much space do you have on either side of the motor tube? I was able to outfit a 29mm motor mount with positive motor retention (PMR) with only 3/16" of clearance on either side of the tube.
You may not be able to put blind nuts on the back side of the lower centering ring, because it is already glued in, but you can install the mount in "backwards." Simply drill a pair of holes into the rear entering ring on opposite sides of the motor tube, right next to the tube. Then obtain two short lengths of all-thread (threaded rod - 4-40 or 6-32 should be sufficient), about 4" long, and epoxy them into the holes while bonding them to the sides of the motor tube. Use a good quality epoxy, like JB Weld, that can handle the heat. Leave enough projecting out of the centering ring so that there will be about 1/2" to 3/4" of rod extending below the lip of the reload case or the end of the SU motor when it is installed. Slip some suitable retaining clips onto the posts, and fasten them with locknuts.
Screen door window clips will work, but for something so close to the motor, I have used another piece of screen door hardware called a "button." (It doesn't actually look like a button, but rather like a small tab. It is actually part of the latch mechanism, I think. There will be nylon ones and metal ones; pick out a metal one that looks suitable.) You'll find it in the same drawer as the window clips. Another type of clip that I have used for tight spaces is a speed nut. You also have the option of obtaining some brass strip from your local hobby shop and fashioning your own clips that are sized just right for your mount. If you can find them in the right size, you could even use "T"-shaped (or "narrow") blind nuts, screwed onto the posts upside down, with a locknut on behind each one to retain it. The flange of the blind nut would act as your clip. Whichever type of clip you use, always retain it with a locknut, even if it screws onto the post. And also get a nutdriver that is the right size for the locknut; it can be much easier to use than a wrench when you are at the field.
Hope this helps.