There are many good ways to mount a motor on a scratchbuilt foamie. Here is a simple and effective method I have been using that utilizes an L-bent piece of metal plate, heavy duty two-sided foam tape, and a plastic gift card. It is strong in flight but comes apart in a crash such that it is field repairable. Also easily adjustable by reapplying the tape and repositioning.

Thin aluminum and steel work fine. I am using 1mm titanium plate salvaged from used ("burnt out") saltwater pool electrolytic cell. This is the component that converts salt into chlorine for disinfecting the pool. $400 new, it's a shame to throw the used ones away. Use for RC!