In most cases, we do not recommend fastening to the arms on wheelchairs.  The Permobil arm is an exception.  It is so powerfully built and stable that it is a great place to fasten a mount to.

We place one half of the monoclamp as shown.

Once we get the other half in place, we can bolt a complete mount to it.

Here we have a CS 20 Arm mounted.

Here we show a fold down mount.

Here - a folded arm.