Fred:

Here is a cut and paste illustration of the flexible air duct utilized in automotive heat risers from the 60's and 70's.
Its still available in various diameters.

You open up the dump doors on the bottom of the air scoop and fuselage to permit air to vacate the engine area.
This solves the problem of an enclosed cowl on an inline engined model. I have edited the illustration to show the
exit fixture is constructed to attach to the light ply frame constructed for the rear dump door. The first one was a
little more than crude and probably confused some people... : )

Cheap to do and so easy...even a caveman can do it.... ;^)



Here is one style of flexible duct from the TransAm These are available in various sizes and wire hoops per inch.