Do I recall correctly, a Rocket has an exit ramp inset into the belly, like the -8?
I built four different cowl panels, each progressively smaller in exit area, all around a 4-into-1. The fourth included variable area. It's just glass. Build what you need.
Stock, never flown. I did send the pipes back to AWI with instructions for shortening the headpipes just a little. That raised the whole pipe set straight up. Compare the pipe in the first two photos below:
View attachment 66184
#1, before first flight:
View attachment 66179
First three exit panels:
View attachment 66178
Raw panel #4, prior to cutting the variable exit door:
View attachment 66181
#4, early on, pressure and temperature measurements underway. Still flying it, about 1000 hours now.
View attachment 66182
View attachment 66183