So today I got around to shaping out the ejection port on the Scattergun drum. I did this by initially using a Dremel cutting wheel to remove a section smaller than the area I needed removed. Then widened it loosely with a Dremel sanding cylinder before using self-adhering sandpaper attached to a ballpoint pen and a tongue depressor to smooth out obvious imperfections.
Switching over to the Ambassador's sight rail progress: I was finally able to get a section of material cut for me. An awesome guy who owns an acrylic fabrication shop near me used his table saw to cut me a nice length of 1/4" acrylic sheeting. Now onto this contratption you see above. The Ambassador's sight rail has a shallow groove that runs the length of it. I clamped my Dremel to my 'work table' (fancy talk for a sheet of MDF laid over the remains of a cinder block grill) and clamped a painter's stick as a guide.
Using only eyeball measurements I used a piece of scrap as a proof of concept. Concept proven.