A thick soft foam plastic does a neater and better and more light weight job.

I worked with a rock-climber-engineer who was working on doing a 3-d scan of people's feet and building the shoe (or at least an insert to the shoe) with a 3-d printer or sterolithography machine. He intended to make a 3d-mold from the scan, then form a rubber-part inside the mold. The shoe would 'exactly' fit your foot, but still have high durometer. Helpful for rock climbing I understand.

I don't know what happened with him or his project, but I can see that the technology that he was pursueing has come-down in price significantly and this might be quite do-able now.

Probably this would work better than sand. If you still think the sand idea is the best, possibly a person could use casting-technology to cast a mold for the person's foot and then mold the foot, then mold the insert from it. Or cast in a material that could be sintered.

But in a shoe, sand would move around unless you could fix it. would be uncomfortable and waste energy. I've had sand in my shoes before.