I was looking around for things to scan for model textures when I picked up an oil filter and had an 'Ah-ha!' moment. Here's a small scan of the end of a Fram oil filter that screws to an engine. I selected and filled the hole in the center with black then selected around the outside and cut to a new layer, then copied that into the PSD for one of my fighters. With some scaling it became part of the texture for the gun and missile ports.

With some imagination it could also be an engine exhaust, a fuel hose connection, a power or data jack, or various other things.
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