Piers - thinking back to my jazz band days - presumably you're holding the trunpet securely with the hand which does the fingering? Most of the players I've seen using the cup, either play a passage with it held to the bell and opened as required to play the wah note - or pass it in front of the bell to get a more subtle variation in tone. They usually had the cup resting, upside down, on a nearby flat surface, so that they could pick it up when required. I have to say, a little goes a long way, but it can be very effective in the right places.

If you want longer arms, I suppose you could hang for long periods from the top of a door... (I'll get me mute).