I am thinking op putting a red dot sight, or similar device on a shotgun for hunting wild boar with slugs.
Who has done this before and can give me some advise?

There are commercial plastic sights made in America. I think Day-glo is the more highly regarded. An issue of Shotgun sports magazine had an article on various plastic front bead optical sights several months ago or perhaps late last year. This magazine is on the web.

I got a sight set with one metal mount into which several different colors of tubular sights could be installed. The green sight has a higher light value as green is closer to the blue, or "fast" light waves, while red is closer to the slower light waves. That is why the Russians (and allies) use green tracers - they might be visible for a greater distance though I think this doesn't matter for many applications.

I have a green sight mounted on a Baikal shotgun. As soon as I get some heavy metal foil or thin and soft sheet metal I will make mounts so I can use the other colors which came in the package. Some of the viewing ends are round and others are pointed. Use whatever has proper contrast for your application and background color of forest or grass, etc. Red would be a fine contrast or compliementary color for a green brush background.

Where will you hunt? I have heard Englishmen on this site complain loudly about French boar-hunting sites. Have you ever considered just buying some plain old fine quality bacon and having a monster-sized sandwhich with some sliced tomatoe (yes, there really is an "e" in tomato - or at least there used to be)? A fine quality rye bread sliced into huge slices is necessary.

I have never tried a red dot on a shotgun, but I am using a cheap BSA on a 22 rim fire pistol and it works great.

Just speculation, but I think it would work well at 75 yards, but could be a problem at close range with a moving target. A 42mm tube would give the best field of view for quick target acquisition.

I've been using the same cheap BSA's on a couple 3" & 3 1/2" 12 ga. turkey guns for 5 or 6 years. They've functioned and held up fine, so far. These are 30mm sights. For something that moves and bites, I'd probably want a 42mm or 50mm.