Hello,
I had a similar issue with my BTCC rear bumper. Someone had cut the exhaust holes bigger than they needed to be on both sides.
To get around the issue I decided to build my own custom carbon fibre diffuser and then cut the holes in afterwards where I needed them. Doing it this way meant that I could have gone with either a
K1 style centre exit, twin exit each side, twin exit centre or just one side exit. I decided to go for a twin exit setup with fins as well.
I will be uploading a build thread of how I made it in the coming days.
Thanks