It'll fit, you may have to do some light sanding... - Magpul Industries Enhanced Ejection Port Cover

I installed two on Aero M4E1 uppers and didn't have great function. I removed the cover (which takes seconds) and then lightly sanded the back bottom (where the exposed ribs are) with 120 grit sandpaper and after a quick clean-up of wiping the dust off, it installed and functioned without issue. The sandpaper didn't leave any unsightly marks. It's a quick fix for a nice functioning dust cover. Better than Mil-Spec generally speaking.

Bottom line: If you are NOT inclined to do some light sanding to make it fit then don't buy this product.