The slot is an even width for the length of it. I'm going to take the frame to my LBS and have them measure the ID of the tube and make sure it's even, etc because I don't have the tools for that.
I'm able to get a seatpost in a few inches, but then the resistance becomes too much and I can't get it the rest of the way in. It's so tight that, when I was trying to push it in, it literally shaved off the outer anodizing on one of my black seatposts...