When I built my first BW (Shu!), I used PVC rods as uprights and screwed into that. That worked but wasn't very strong overall though in all fairness, the boxy shape didn't help and PVC itself is not a good material for this - HDPE is much better.

I opt to use threaded inserts into square rods these days to bolt two panels together. Barrel nuts are another option but at the BW level, I find the inserts work well enough.