horizontal under the seat isn't really an option for me, as accessing the crutches would be super difficult (for attaching and detaching them.) it is a carbon fibre folding chair, so the tubes are non-standard.

I would like the crutches to be vertical, or if slightly diagonal angled under the seat, easily reachable by me if I turn at my waist in the chair. The attachment must also be 100% rigid with no swaying, as that would drive me completely to distraction.

I'll probably print something like this semi-rigid clip that would have a little bit of velcro over the end to keep it from popping out if I hit big bumps. I would have to print them to get the angles and tube shape to work, nothing off the shelf has the right angles.