Greetings ....
I thought I would wait for the gents who actually know what they are doing to post first before I ramble on aimlessly...
I thought I had seen commercial rigs with sight tunnels as JRice mentioned...
here is one, from Bianchi trademarked as "sight channel"
"Model 1810H Clearwater™ Cowboy Holster"
If I were to try building one for a flat top like yours, I think I would try to build a three-layer extra thick grooved gusset for the sight to ride in...
but I have no idea if it wouldmake drawing smoother than a formed channel or hang up even worse...
good luck and let us know what you go with !
yhs
prof marvel