Athilex MC4 extension leads in black, made to length from 1m to 20m: 4mm² with IP67 connectors as a single lead or as a set of two, and 6mm² rated 1800V, again single or as a pair.
Cross-section is about the run, not just the current. On short jumpers between panels 4mm² is enough; once the cable has to reach from the roof to the board, 6mm² keeps the voltage drop down over the same 20m. The 6mm² leads are the ones rated 1800V.
The two-cable sets are the sensible order for a string, since they give the positive and negative run in one go and in matching lengths.
