A small piece of plastic tubing would work. You'd just have to experiment with size.

I thought the whole philosophy behind the ring and post style sights was to get a faster target aquisition based off the premise that your eye was naturally going to center up your target in the "ghosted" circle without you having to even think about this. With that in mind, all one should be really concerned with is the front post's location. The post/POA should theoretically gravitate to the center of the circle anyway.

Ring and post is not the same sighting idea as target-style peep sights. If a peep sight is what you are looking for, I'm sure someone sells a picatinny mounted version for AR's.