Wow you guys with bigger planes have a lot to worry about!
Here's what I've been doing. 1106 motor, really tiny. X-mount made from a servo horn. The plywood is 3/32". 1/16" would be plenty strong enough but I want more wood for the screws to bite into. The firewall is .8" square. Gorilla Glue to mount the firewall to the plane, drilled to drive pretty large "fancy" toothpicks through. The toothpicks are CA'd to the firewall and Gorilla glued around 3/4" into the foam. You can just see the end of one there. I will never have one of these fail.
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