I had to repair a 7d (water intrusion). After repair, I have found a button (the liveview) without function. I disassembled, and found tha the rubber is too short, so you can not push in enough the plastic button.
To purchase a new piece was not possible, an d I wanted to repair it immediately.
This is the original.
I started to search in my treasure-box. And I have found an old 300d, with the following button.
The material is a kind of silicone rubber – not possible to be glued, not to be melted. So I cutted the margin, then I made two small wholes in the new button.