When ordering motor mounts out of town (mail order) it is best to do this trick.
The motor mount might not fit if something like this is not done.
It is too costly to ship a complete SS fire wall steel sheet.
Don't use aluminum as it expands and contracts with temperature too much.
Mylar is problematic. Not precise enough.

Get some 26 gauge galvanized steel strips about 5 foot long and 1.5 to 2 inches
wide from your local heating and cooling duct fabricator.

Tape the strips to your firewall in the pattern shown and match drill
 the bolt holes from the back side. Put them into a mailing tube for shipping.

This insures your motor mount will fit when you get it back.

Paul Lamar