As far as stretching the sight clamping pressure... with it off in hand, I simply put a wide blade screwdriver in the tabs and twisted a bit.Don't think you could go over the top without scratching the barrel.
I hold a file laying on the edge of my workbench. Take sight in other hand & rake the inside flanges on the file just a little . It is good to measure before & after, so you don't take too much off. You could bevel out the leading edge, but not necessary.
Drilling, tapping and two screws cost more than two hunks of plastic.
I have seen a couple of people , [ yes they were good repairmen ] place the wedges in the sides, sit the sight on top of the barrel , then whack it with a rubber/wood mallet. That SKEERS me !