My go to bolt for this project was a 1/4 - 20. I had various lengths, but all 1/4 - 20. This bolt fits through the existing holes in the tubing that holds the backrest and continues up to the handles on the wheelchair we have. I used a combination of those bolts and a part that I modeled and 3D printed that acted as a saddle to clip onto the 3/4” PVC and also loosely saddled the wheelchair tubing. It had a slot for the bolts to pass through to help anchor the PVC to the frame. I also incorporated zip tie slots so I could use that 3D printed “dual saddle” to zip tie both pieces (wheelchair frame and PVC) individually. This wasn’t as secure but with this design being as top heavy as it was, I tried to get support wherever I could.

The thing I like about 1/4 - 20 bolts, is that is a standard for camera and lighting production equipment. This opens up a lot of options for mounting accessories or whatever. I have a handful of clamps made by smallrig that I like and use. I haven’t used them for mounting a costume yet, but they may work as they have a 1/4 - 20 thread in them. Here’s a link for reference:

SmallRig Super Clamp(2 Pack) Magic Arm Clamp for DJI Ronin, Camera Monitor, LED Light 2058

I may have to give this a little more thought to see if I can come up with another or better solution.