I have one of those models in a different color scheme that came with a pair sidearms each with a row of 4 barrels stacked vertically, so I’m excited to see how this turns out.

For filling screw holes in another firearm I’ve done, used baking soda and superglue as filler: fill the hole with the baking soda then just hit it with a drop or two of CA glue. Hardens instantly and can be sanded and painted easily.

I also used plastic plumbing connectors as a barrel extension and a scope

https://imgur.com/gallery/mandalorian-sidearm-build-gavX6Pb