Not much of a modification, but this is just a little idea to help stop the plastic on the ICS SIG series bolt handle from breaking.
Months ago I got my 551 & as many before me, I broke the bolt handle. Mine lasted for about a dozen times of releasing the bolt catch before it sailed across my apartment (much to my cats amusement).
After sourcing a replacement, I wanted to try and avoid that from happening again ......
I used a small piece of a foam/rubber stress mat to soften the impact. Cut roughly to shape and reduced the thickness to about an 1/8 of an inch. This was then glued inside the receiver.
I also cut about five coils of the spring that pushes the bolt forward

Since adding the pad and shortening the spring I have released the bolt catch at least a hundred times. I'm happy to say the new bolt handle has yet to show any signs of cracking or breaking.
Different materials might work too, this is just something I had at hand.