To stop a motor from moving in a shunt...
Make sure that you have steel washers under the head of the motor screws, the larger the better. On some cars you can't use one any bigger than the head on the lower screw as the lower pod plate sticks out beyond the motor mount and a large washer will foul it.
Find yourself some used 600 grit Wet 'n Dry paper. If it's new then get something like an old diff washer and rub it across the paper 30 or so times to knock the edge off the grit. Alternative is 1000 grit, but since one never uses that for anything else its a bit of an expense!
The problem is that the surface of the anodising has a low coefficient of friction. Take the wet 'n dry paper and very (VERY!) gently rub it across the motor plate where the motor fits against the plate (the inside of the motor plate) so that you take the shine off the anodising, but don't take the anodising off. It should look like a matt finish now. This greatly raises the coefficient of friction making it harder for the things to slip.
Now do the same on the motor. Again, take the shine off the anodising, not the anodising off the motor plate.
You can be a bit more aggressive than you think once you get the hang of it. The surfaces should look matt and scuffed, not scratched and damaged. If you do knock the anodising off the odd corner don't panic, it won't do any harm.
Now clean both surfaces with some brake or motor cleaner and leave to 'dry'. Re-assemble as before. Do not keep tightening the screws to avoid the problem, you will damage the threads in the motor plate to the point where they won't hold the motor in properly, or will strip.
The combination of the steel washers and the scuffed surfaces should prevent the problem happening again. As, of course, will driving better and not hitting the barriers in the first place!!
I hope that does help.
PS - take those shims out! They will not fully support the motor which makes it more likely you will bend a motor plate in a shunt. Motor to plate only please!
