I like that idea of the copper washer. I guess you could find such a thing at a plumbing store, but I see also that
Misumi does them in any size/thickness you want (love that place!). Interestingly, to get the 6mm^2 you'd need, the washer can be amazingly small: 4mm ID, 10mm OD and 1mm thick would give you 6mm^2 cross-sectional area. Of course, your solder is going to supplement that, so definitely no problem handling the current even with the smallest washer you can find.
This has got me thinking what I can produce with sheet copper on my CNC, which
finally arrives next week
