drill you transmission and pop the magnet in it with some threadlock or super glue or place the magnet on the front driveshaft with a spec of superglue and use some electric tape around it. make sure the tape is so taunt that the magnet bulges through the tape. I did it this method and the thing is still there 8 months later with no sign of giving.

at least this way you can remove and place on any other cars that you have without f-cking up your transmission. This method though doesnt have a accurate readout due when on bumbpy surfaces dues to wheelspin but on road itt works cool.