I've used the one that came with it for quite a while with no problems. I bought another chamber flag that has a ring and 3/4"x3" or so red flag on it, so that it was easy to grab. However, the nylon part that fits into the chamber is almost 1/2" wide and almost the same width as the opening.

So it contacts about 80-90% of the bolt face. With the extractor sticking out (and would fit in a slot at the breech face) it contacts the chamber flag. Hitting it hard enough, will dislodge the extractor.