Tell us how heavy it is before we go any further Graham.
Most cycle racks will take about 60kg. Much heavier than that and it's not the way to go, specially since the weight of the enclosure has to be included.
You may need to fit a towbar and use that to support a carrier for the chair, but again the weight could be critical. I think (but don't take my word for it
!) that most towbars will support about 150kg . . . IF
there's enough capacity on the rear axle loading, AND IF
your overhang is not so long that it will dangerously reduce the load on the front wheels and make the steering too light.
If you consider the towbar solution, I'd suggest you contact Messrs Watling Towbars. They will do all the calculations for you and if it isn't safe they will refuse to fit one - which is very reassuring, but not true of all fitters by any means!!!
They do a range of carrying platforms as well, and I'm sure they would be able to accommodate your gaffer's wheelchair.