So I now have all the materials for Mjolnir. Polystyrene for the hammer head, wood for the handle, leather for the handle grip and plastidip for coating the hammer head. I'm short the amount of leather needed for the loop at the bottom but I might use string or something for that.
Crafting
Initially I tried cutting the polystyrene with a craft knife but that didn't work very well. I also tried using a saw and a carving knife. These were also all terrible ideas. I finally settled on heating a bread knife over the gas hob. This cut through the polystyrene like butter but if it was too hot caused more to melt than I wanted. Sadly we end up with this: