Have you tried a bosun's chair or breeches buoy?
If you have a medical equipment supply company nearby, check out patient lifters.
Hoyer makes one of the best. They have different sizes & styles of slings, manual
or motorized operation, are wheeled. dismantleable and portable, and if you can get
a prescription, Medicare should pay for some of it. This site shows what I mean:
Patient Lifters - Home
Standard Disclaimer: I have no connection with the above-named companies. I worked
in the medical equipment business doing fitting, installation, repair and customization of
many kinds of patient-care devices.
Best of luck to you.