My setup inculdes two IMO's. I wanted to have one pick up magnet on each side in order to make sure that if one side gets damaged I do not loose both pick up magnets.
I have used a man made bracket that bolts behind the rotor bolts (in the inside of the hub) and there I use an M8 Touratech screw magnet. That is on the brake side.
One the other side, I just drilled and tap the hub and put the pick up in the fork bolts using another man made elbow plate.
Can take some pics if you want to show the assemblies.
Of course as BajaDad said, if you have the second disc removed it is very easy to bolt a magnet there. Very nice and clean.
For Magnetic Touratech bolts see here,
Although you can make your own if you are mechanically inclined with better magnets that do not demagnetize so easy over time ...