I think aluminium would be ok. 8mm wide, maybe 4mm deep ie rectangular would be just as good as a rod and probably easier to source. the hole is approx 4.5mm wide, 5mm at the max. try 4 or 4.5 first and widen if necessary. Better to stay 8mm wide (not 10) as too fat is too difficult to fit into spring number 1 area - it is tight. also wear gardening gloves or something with tough material as pushing against the rod or bar hurts your hand - the spring pressure is quite high.