Others have discovered the magic of TUNGSTEN
Okay, so they aren't really useful for derbies (except perhaps outlaw derbies that allow very heavy cars). And they're really expensive if you're buying tungsten as weight. But other than that, they're pretty cool.
For a "how to build a faster car" talk as part of our derbies, I've been thinking of building a set of blocks, each weighing the same amount, but made of different materials. That way I could show the kids what 1oz of balsa looks like, what 1oz of pinewood looks like, what 1oz of tungsten looks like, etc. This could also be part of an intro physics unit.
But using one of these tungsten cubes for that demonstration would be difficult, just because I'd need much bigger blocks of everything else. A 1kg block of balsa? Maybe not...