I screwed them to a bit of thick plastic I had lying about- just drilled holes in it for the mirror's own bolts. Worked really well, though you have to be careful when leaving the mirrors to dry as they're not very stable like that. I managed to drop mine on the dusty floor when I did the primer coat and had to cut it back and start again 
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"We are the angry mob,
we read the papers every day
We like what we like, we hate what we hate
But we're oh so easily swayed"
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