You may look at light tools, they also have very good stuff, I use these on my chimeras. Never seen the one from broncolor, so I cannot comment on it but I would avoid gaffer tape on the umbrella. The para has those little balls in the front you could easily use some C47 to fix a gel, diffusor or grid. But go and ask at Briese and you´ll consider broncolor´s eggcrate a bargain ;-)
Thanks Ulf, I'll check it out.
In theory if I can find a grid which has about the same dimensions I would be able to attach it to the pins that stick out of the para and simply stretch it to keep it in place or use gaffer tape, even if you don't recommend that

It would be interesting if someone else found a grid from another brand which could just be plug n' play.