I have done the water jet test, it is a success. Waiting for the rain for final test
It's sounds like your on to something there, hopefully a slightly bigger plate will sort it ! .Some goods news and bad news.
The good news is that it does work.
The bad news is that the reason of splash in the back has 2 root causes. The first root cause is definitively the splash coming from the water crashing on the reg plate. So my DIY works.
I think the other root cause is the water splashing between the reg plate and the rear light. I did put a 20cm wide aluminium sheet, so I can only imagine it is not wide enough.
I ll order a 25cm and a 40cm ones and will let you know.
In the meantime, I saw this mudguard much cheaper than the one you had in mind:
Rear Mudguard for Suzuki GSX-S 1000 F / S Katana hugger Zaddox LW6 black