First, it's rare that a cover mount will hold zero. But before I get yelled at, some do. I have one. I also have three that won't and no amount of filing has helped, even though they seem to fit tightly.
This mount, however, is as solid as a rock and holds zero perfectly. It's a B-Square and it requires that the retaining pin for the cover latch be removed, but nothing else.
Martin is right about a cover mount holding zero. Some do, but it's not something I'd bet on.
A pistol scope mounted on one of the Tapco handguards seems to work well. They don't always work either, but the failure rate is lower than cover mounts.