Fitting the sniper knee is a right of passage, much like the S-Trim on your bucket. I think mine was 4 tries.
As has been said, look at the Centurion threads. Line it up as best you can on the sides, get it level and lined up in the front.
Hot water baths can work miracles.
Adding foam padding behind the shin on that side is almost a must to keep it out of the way.