I think the only thing to do is drill a hole in the mirror and set it in there with epoxy
But I am going to stick it in a little block of foam such that it will be in contact with the tube and stick it on the tube but near the back and on the top so everyone will ask "what's that?" Together with the remote temperature sensor that came with my camping fridge with the readout in the van so I can monitor temperate in the observatory all the time..that will be handy...
Alex