Do you have unusual shaped or sized tubing that you can't mount a standard bracket to it? Or is the bracket to chair mounting somewhat standard but you need to hold multiple tubes to it at a time?

If they're being carried horizontally there's less need to worry about the chair to holder part slipping since it's not load bearing... Here's some examples of where my mind's going, even if the particular items aren't the right fit:

Velcro wraps on both sides

Clamps with holes

Clamps with slots

Rubber lined clamp

For odd shaped tubing or creating your own clamp/hook/loop design:

Roll of hose clamp

Pipe hanger strap

You could run velcro straps (or whatever else will hold the crutch parts together) through slots, bolt them through holes so they'll never slide out of the mounts, then use some grip tape around the middle parts of the velcro that would be holding the crutches and not sticking to itself?

Maybe even rig up a Cam Jam or similar? (I don't have any of those, I just saw them and thought they looked useful for something...)