Honestly, I wouldn't bother with a guard like that. All that ends up happening is the taps on the caliper mount end up breaking. For a proper rear disc guard, you need to replace the whole caliper mount with one that has the disc guard integrated into it, like the Scotts unit.

This guard has had quite a few hard knocks (front on,mind you),and has the dents etc to show for it.I assume the guard would be weak getting hit from the side though as all the leverage could pull the 6mm bolts out of the holes.
Touch wood, its been fine and done its job so far.
Cheers Mark