I found the Sgt. Peep woefully small at 0.04".  So I installed a spare 1902 sight that I didn't mind altering and enlarged the peep to 0.07".  That's about as big as I could go without damaging the plate.  It made a WORLD of difference in sight picture!!!  Bear in mind that you'll also need to enlarge the notch.  Otherwise, you'll be looking at the notch through the peep.  I used a set of wire gauge drills to accomplish the task, and then used a tiny rotary file in my Dremell to carefully clean up the hole and edges (remember, a twist drill does not make around hole).