This worked so well for me that it's amazing.

The plane ( and ME! ) now stay clean.

I didn't need any special silicon tubing, A muffler extension as you pictured, worked perfectly.

I cut the holes and CA'd the entire area in the wing from the opposite holes, to add strength.

I epoxied a metal grommet ring ( used to grommet sail cloth ) with a bit of coaming on the top and bottom.

After installing the muffler and extension I used canopy glue on the extension to seal the wing opening and prevent any accidental leakage of fuel into the wing.

If anything my engine seems to be running a bit faster now and my plane is carving BIG arcs in the sky.

At the field I've had several people marvel at how ingenious this is.