The "trick" is getting the thing into a good, solidly braced, "no-bounce" set up.
Use wood blocks, a padded vise, or a buddy to help hold it steady, and put wood support blocks under the sight to brace it firmly.
If things move or bounce, the sight won't move.
If necessary, make a sight drift from a piece of brass bar stock.
My favorite is 1" wide, 1/2" thick, and 6" long.
If necessary, I shape the end to clear the sight.