Lello,
As far as I know Duncan is the latest purchaser of a similar bracket so he may be able to post some info.
Never tried a white rain brolly to know if it will work.
Of course you could always try lining it inside with kitchen foil and using a piece of garden fleece over the front to provide diffusion.
Attatched is a set of pics from an earlier experiment with my silver brolly set up as in pic 1. White brolly v garden fleece as a diffuser pics 2 & 3.
On portraits you will need to think how the catch light in the eyes will look. So a bit of experimentation needed if you are doing it on the cheap.
Maldives ........... Lucky devil.
Don