If all else fails, buy an inner tube for a wheelbarrow wheel ( 8 x 4 ) and inflate to about 5 pounds / square inch. Doesn't need a lot of pressure in it.
Put it in the chair with the valve facing down!
It may not look elegant, but will probably do the job, and last better than a more expensive "medical" product.
If it looks too utilitarian, make / knit a cover for it.
Army horses loose on London streets
Angela Rayner lashes out and calls Sunak “pint sized loser”.