Upper Krust's "Immortal's Handbook" has pretty darn good rules for making golems of any material you can think of, you just need to know a few of the properties of the material, such as weight / density.
I'd probably just use the Dragonflesh Half-Golem template and make slight changes as necessary. There isn't a mithral one that I know if, but Dragonflesh comes pretty close.
Use this for all your template needs. Take a look at the Dragonflesh one, and change what you need to, *nod* (even has wings for flying speed! hurrah! )