Looking at the motor picture , you need to fill in the gap between the motor and the body , with a rubber "gasket" between the new pieces and the motor tin.
The problem is keeping the hot air from below the motor away from the top side of the motor which needs to have cool air going to the cooling fan.
Look at how a stock 356a does it ., part 26 , some are in 3 pieces, left, right and center that unbolt , later ones are 1 piece and welded in .....
Good Luck