[Joachim]

The most dense legal material (other than the battery and maybe the motors, which are already positioned) is probably the steel of the Tetrix axles and the Tetrix screws. Neither is very cheap as ballast, but based on rough volume caclulations, it seems the 1.5" Tetrix screws, with their heads cut off, would pack closesly and be similarly dense to a bunch of Tetrix axles, but at a lower price per unit volume.

If you want to use axles, the 250mm Tetrix axles are cheaper than the 100mm ones, by volume.

Of course other non-Tetrix screws would be cheaper, but you can't use them unless they are the same thread type, and even then they cannot be used except as fasteners, IIRC.

Maybe you could find some identical in every way to the Tetrix ones, but for a lower price, and maybe that would pass the rules.

I suppose you could check the price and density of the allowed (steel) wire rope also, but it probably isn't a great candidate for ballast.