The additional ply doesn't have to be very thick. The real stuff stands compression excellently. I used 1/16" on the new H9 Corsair and the same has just cured on the H9 Spitfire.

Yeah, adding thickness makes the engine stick out more, but you simply subtract the added thickness from the suggested thrust-washer-to-firewall measurement and you're good to go.

That Great Planes motor mount is good enough that I use it on the H9 ARFs. I find it's easier to use than 2 piece mounts, very much easier to use. And it does distribute the compression over even more area than the 2 piecers, but LitePly's ability to stand compression varies a lot more than other plywoods.