Hey @tetraplegic - welcome to the site.
It would be helpful to get an idea of the where you would need the gimbal mounted relative to your chair - e.g., what sort of distances are involved from suitable mounting spots on the frame of your chair to where the camera would be located. Would also be helpful to get a rough idea of where you are geographically located, as this will probably require some local help.
The commercial standard for heavy duty applications like this is probably the Rehadapt Monty 3D: https://rehadapt.com/product/monty-3d-plus-hd. It’s a very well designed mounting system, and the price reflects that. The maximum listed capacity is 13.2 lbs.
None of the current Open AT mounting solutions are intended to carry this type of load, so this will require some design work. One solution I have seen for a similar challenge was to use bent/welded piece of metal pipe, though even that had more movement than desired. It will probably be necessary to connect back to at least two mounting points to provide the desired stability.