I saw the photos of your DerJet Prowler and Rich (RCISFUN) gave me your contact info.
I'm scratch building a 1/7 scale Buccaneer (build thread on RCU) and your engine installation is very similar to what I'm considering.
To help me out, could you answer a few questions;
What are the inner and outer pipe diameters?
What is the overall length of each pipe?
What would you estimate the pipe turning angle to be at that rear bulkhead?
I have a DerJet Hunter with over 100 flights on it (Merlin 140 powered) and I love it. It is my go-to weekend flyer - I prefer it to my Ultra Flash. Great support from DerJet too.