I did it
I bought some new blades for my hacksaw, and it took me 4 hours to cut 2 brackets of the size of a small English sandwich. The result's not bad at all. When I have time I will polish them.
I'm very happy with that because they look tough, and I think in case of a crash I will just have to replace the SSie rearsets, which should be very cheap...
Anyway, thanks all for your help. I'm sure the result would have been much better done by anybody else, but I'm quite proud of me because I did them. I think that's the first time I build something for my bike (ok, except this
http://www.ducatisportingclub.com/xm...6728#pid151284 
).