In Cochrane I started by going to a car mechanic I had already seen when I entered town. He couldn’t weld my rack and according to him there’s no aluminium welder here. He had another bike rack out of steel, but there’s no way that will fit my bike because of the rear suspension and generally non-standard setup. Then he suggested using strong hose clamps to hold the rack in place and sent me to a hardware store.
I got a few clamps there, but I already didn’t like them much when I bought them. Indeed I found out they won’t work for me. As I was trying this at the hostel a motorbike rider arrived who also took a look. He suggested to just use strong zip ties or otherwise use small steel L-brackets. But that will have to wait for tomorrow. Looking at my bungee cord fix I think zip ties, applied correctly, are indeed likely to hold and it’s easy to carry spares in case they break. Update: I had contact with the bike’s manufacturer and they also suggest a fix using hose clamps in combination with a piece of wood, so I’ll try that soon.