Ferrina fitted a lotus style rear undertray and that helped but it won't work with the BRAB lower rear spoiler.
May be worth investigating though
JJ
Ferrina sold me his diffuser as he wanted to take it off and fit a towing hook.
It's been fitted to my car for quite a while now... I can tell you this -
1. It's a PITA to fit.
2. It doesn't noticeably reduce the amount of dirt on the back, it just redistributes it. The difference depends on your speed.
3. Nobody tailgates me on the motorway when it's raining
4. It tightens up the rear when cornering and I like that.