I did this with a holster for my Ruger Super Redhawk too and it works out fine. 
I purchased a holster to fit my Ruger GP-100, knowing I was going to have to alter it for the red dot sight. I cut the notch just enough for the picatinny rail to fit into the holster. Then I burnt the edges and sewed them closed with sinew. 
It's easy to do with a half hitch stitch, stays in place and looks decent too.
I purchased a holster to fit my Ruger GP-100, knowing I was going to have to alter it for the red dot sight. I cut the notch just enough for the picatinny rail to fit into the holster. Then I burnt the edges and sewed them closed with sinew. It's easy to do with a half hitch stitch, stays in place and looks decent too.
