One of my favorite engine tips is those black composite plastic engine pads as sold by several manufactureres (Sig or Brodak). They are
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8" thick and ideal for profile fuselages. Make sure you get the one without the built in offset. Sig's are not offset.
I used one as a pattern to make an inlay jig for my router. The jig is nothing more than a 4" x 12" piece of
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8" basswood with a centerline drawn down the middle. The jig is then either tack glued with hot melt glue or clamped onto the fuselage, making sure the centerline of the jig coincides with the centerline of the fuselage. Then, using a ¼" or smaller bottoming bit in your router, machine away the ply doubler material.
You'll get a perfect fit to drop in the plastic C pad, your maple engine bearer surfaces will be trued up and your engines will never crush the doubler - cuz there ain't one.
I've gotten many compliments on my steady engine runs on my profile ships. This is my little secret...