Is it right that it can be enchanted like any regular armor?
Hardness works against spells, too, and most spells deal only half damage to objects. So we need either to make the armor "immune" to attacks or optimize its values.
Hardening with CL boosts will help. CL 20 = 10 bonus hardness and pretty easy to get once you use transfer essence (spell that grants half the CL of another caster).
Also, after errata any magic enhancement bonus increases hardness by 2 times the enhancement (10 for +5 armor) and the HP by 10 (50 for +5). So we get with ease an armor with 30 hardness and 150 hit points. Crafted with the feat from #358, you get +4 on saves versus acid, rust and disintegration and +5 hp. Dwarvencraft for +2 hardness, +2 saves, +10hp. Hellforged for +1 hardness, +5hp.
The total would be 33 hardness, 170 hp. It can be improved even more with a greater CL, but I think this is a good start.