No modifications at all on the inside of the bags. I still need to plug the original holes. I picked up some black plastic plugs at an an auto-parts store — the plastic Help brand discs used to keep door skins in place — but have yet to find a size that works. They're barely noticeable anyhow.
It took a little fiddling to make sure the fender bolts were evenly distanced from the fender so that the bag would drop right in. But once the positioning is set, the bags drop in and come off pretty quickly. I kept the left bag on the factory bracket while determining the position of the right bag. But even without that crutch, it would have been pretty self-evident based on the position of the shock and the right mufflers.
I'm hoping that my as-needed use of the bags will help keep their shape. Truth is, I don't need them every day. The Easy Brackets are a perfect solution.