I don't have experience making such joints, but I think they might be overly complicated. You need to pierce the mating tube at two spots and then cap the protruding end. For a stem (or tandem) it makes sense because you need the bolt (or seatpost) throught and the second tube has a larger diameter. In your case a simple mitred joint, with brass, would be as strong.
Your tube dimensions look okay, but stay with a 1 mm wall (you probably won't find a 0.5 mm walled tube anyway). For the pierced mitre the tube needs to slip in without resistance but without gaps. The size of of the drilling cutter will depend on the rigidity of your setup. With a hand drill, it would be next to impossible to keep the two holes of each mitre centred on the tube, not to mention keeping the series of holes in phase on the larger tube.
Here are the reasons why I am thinking about using this kind of joint:
- could not find such lugs
- with classic mitered joints I would need to miter 4 tubes on both sides to exactly matching length.
- I am not sure if it is con or pro, but in difference to the bicycle, on this trailer forces would be acting in different direction regarding a joint i.e rotating
For the holes drilling, I have hand drill with stand to mount it in, and I was thinking of creating some fixture to hold the tubes in right position while drilling. But maybe I could also give tube to some machinist for dirlling.
As for dimensions you were right, ones that are thick 0.5 are only in catalog, but not in the stock where I am buying.
Someone else asked what do I mean by non bicycle tubes, I was referring to steel tubes bought at common local hardware store. How are they called in english I am not sure, here they are referred to as precise thin walled non welded steel tubes (in my rough translation).
Thanks to all for suggestions.