After a lot of googling, I found
this.
I tried both proposed solutions. First, I turned the foresight upside down, put it in my vice, and peened its little feet with a centrepunch. It raised dimples, but it wasn't enough to stop slliding.
Then I slipped a screwdriver in between its feet and separated them slightly, probably much less than a sixteenth of an inch.
That pretty much did it. It slides in now and I need the centrepunch to move it. I'm not 100% sure recoil won't move it, but I'll bet on it since recoil is completely orthogonal to the axis on which the sight slides.