Finally I tried to cut the clips, and it was a complete success I cut it with a metal grinder and sanded it after. Thanks for your advices !
And after ... I tried to fill the shims with ABS paste. What a disaster. I tried to make it larger to soften the transition, but now I just have a large band that I can't remove the scratches made by sanding. I trired Novus 3, 2, 1, alouette ! How the hell can I sand it to make it even with a tape to protect the polished part ??? I finished sanding it with a 400 sandpaper, but I think the larger scratches are still there and the 400 one didn't remove them entirely. Here is the result :
After this disaster, I tried it narrower on the other side :
Oh, and I began removing the black paint on the boots, but I ran out of deglazer on the second boot ... So I have to buy another bottle tomorrow to continue.