I've seen some troops without a flip-up BUIS mount the entite top carry handle and rear sight assembly on the bottom handguard rail of their M4s/M16A3s, just in case their day optic went south.
It looks a little clunky, and I'd have some doubts about how close to zero the handle is when it goes back on, but hey, if that's what you're issued and it's the best you can go with, go with it.
I have a very slim, light Magpul BUIS on the back of my rail. A very critical item for me is to keep easy traditional method access to the charging handle. That is, a standard handle grasped with the right ring and middle fingers. I’ve seen scopes and BUIS’s that really block easy access to the charging handle and I think that’s a big mistake. The answer seems to be heavy-duty handles with big lugs haninging out to the left side, for smacking back with the left hand, but I’m too old a dog to learn that new trick.