I polished the barbeque grill. *sigh* Looking good, Mads. Good you have some decent weather to work in. Today and tomorrow are the first above-freezing days this whole year, and the winds are blowing 40mph. I may start dorade work, anyhow.
For the holding tank: you can find nylon (Delrin?) tank/bulkhead fitting, usually with barbs, and many hardware or marine stores. Same idea as this:
These extend some distance into the tank and secure to the tank using a ring. Downsides are difficulty sealing, risk of ring backing off, and tank won't drain completely. Cutting off the excess spigot inside can reduce the projection, but unless you put the outlet in a sump, there will always be 2cm of sewage in the bottom of the tank.
A lower-profile and more secure option puts the retaining ring on the outside of the tank:
Easier to get a good seal and to re-tighten if needed. These are also sold as thru-hull outlet fittings for bilge pumps, like this:
You could also install a two-piece bulkhead fitting with internal threads, then install a hose barb into that, but that's bulky and complicated.
The simplest, strongest, and most leak-proof alternative is to glass a permanent outlet nipple right into your tank. You can use fiberglass tube or PVC pipe, roughed up with sandpaper. The key is a smooth lower section for the hose to clamp to and a generous amount of filleting and taping where the pipe exits the tank. You want to prevent any sideways forces from levering the pipe out. That's how your rudder tube and engine shaft log are constructed: a tube glassed onto the hull.