Thanks I get the idea now.
I might have another solution, I see there is a 6 and 7 gallon version of the fermonster.
I have the 7 gallon version and the 6 gallon one is apparently 2 to 2.5 inches shorter.
Hopefully it's big enough to get a full 5 gallons of beer for the keg.
I found a shop that is selling them online for 16.99 € in Germany- stock clearance.
Thats about half price compared to other shops, the only thing is they have no lids without a hole left.
I could use the lid I have already prepared but maybe I want to make a second one.
BTW looking at the BYO article I did it slightly different, I added a 3 inch dip tube to the gas post - cheaper than a barb and less screwing around but maybe i might want to add a clamp around the tube just in case.
Edit: just saw your point about the grooved locknut - mine is smooth but appears to be airtight.
If I have problems maybe I could add a grooved washer inbetween instead of replacing the nut.