@tombusey thanks for checking whether this fits, or not. However I am thinking to place the battery rather lower, between the legs. In either case this is probably going to be a trial and error process, and see what works best, when the whole setup is on my back.
Here’s the battery with the first layer of nickel strips:
and a closer look
The serial connections will definitely need a second and third layer of nickel strips welded on top, to support the high current flow, I also still need to add the BMS and the cables to connect the whole thing to the openppg.