Really Tam would be the right guy to do this. Some welded stainless steel would work perfect. The problem is you would be essentially adding more tailcone to the engine, what does that do for thrust loss??? Maybe it's just like adding a small pipe, meaningless. You're only gonna be able to do this on outside mounted engines (sorry, a shockjet would not work either).

You don't find this device on any fighter jets that I'm aware of.

Then there is also the AMA rule that excludes this to contend with.