Trim A evenly all the way around but stop short of the pins and the little mound that holds 3 pins.
Avoid cutting anything under A. For example don't cut tabs B or C. Note I got carried away and scraped C but it still works.
There is no reason to cut lower levels like D. The EF-S lens does not extend very deeply into the tube.
Modify at your own risk. Cut the wrong thing and the tube may not work.