If a radial mount, I can tell you a couple of ways NOT to do it:
1) DON'T put metal posts (nails) through the firewall from the back, then slip the engine over the posts and secure it with rubber bands around the posts and stretched back to a dowel. Looks like it would work good, shock-absorbing quick-change mount...till you crash. Then the engine rocks on the posts, as intended, and breaks its lugs off, which was not intended. Wasn't satisfied with destroying a plastic backplate that way; I had to go and find it would break a metal one as well.
2)Then I tried putting the nails, not through the firewall, but just through the lugs from the back, so the heads rested on the firewall...now the posts would move with the engine, no strain on the lugs, right? WRONG. Took a little longer to break, but still did so from the strain of the rubber bands trying to pull the posts sideways.
But that ain't what you asked, is it?
--Ray