I have a 17" longacre mirror on my car, and rather than using longacre clamps to attache it (it would not easily fit between my rollcage and windscreen, I chose to attach using a 32mm P-clip and an L-Bracket you can buy in a DIY store.

I have been asked a couple of times how i attached my mirror to the rollbar, so here are some pics, and some instructions on what i have done.. incase you might be interested.....

List of things required
1 – mirror
2 – 2* 32mm p-clips
3 – 2* L-shapped brackets (can be got from B&Q, they are the kind you use to fit a kitchen to the wall)
4 – 4* nuts and bolts (and 8 washers) which fit the mirror bracket and the p-clip
5 – 1 drill bit and drill to enlarge the hole in the l-shaped brackets to fit the bolts

instructions
Basically get the brackets, and make the holes the same size as the bolts you are going to use. 
- Put the P-clips on the roll bar
- Put the L-shaped bracket on the inside of the p-clip and put the bolt through (with washers) through the l-shaped bracket and p-clip
- Now put the mirror arm to the other end of the L-shaped bracket and push the bolt through the L-shaped bracket and the mirror arm
- Do the same for the other side and tighten all nuts and bolts.