Well, there's always just irons.
Or a scope:
Scopes are not ideal up there, but I've always kind of had fun shooting them that way.
I know it's not an A2, but what the heck.
I gotta believe Colt scopes were used that way. But I don't believe I've ever seen a pic of one of that Era. Only on 20" guns. There was a dude who recently posted in the M4 block 1.5 thread that had a Colt scope on top of a detachable carry handle. It was pretty sweet.
There is also the Hensoldt's (Blitz's maybe they're called? I think that's what it was) Joescuba used one and posted a pic of a rifle he built as a tribute. I'm not sure if I have a pic of it. But it looks a lot like a Colt scope but longer towards the back.
Or Pat's set up... Which I'm not sure I'd like at all.... That's just getting really high.... (early eotech's / holosights)