Only a small piece of padding stiff foam would be okay to loose the brace.
With this design, you would not need to hold the nuts, the slots just hold the M4 nuts in place.
The brace i am using now would be stiffer though. It's a stock L profile, the horizontal piece holds the motor, and the vertical pieve presses against the lower aluminum skid. (so it can't go nowhere without damaging some tough parts.)