knit slippers

[precariously.livejournal.com]

I'm going to knit a pair of slippers, like the one shown here. they'll be knit without the straps and sort of ballet-flat style. unfortunately, I'd like to wear them outside as well. how can I sole the shoe? I was originally thinking glue and some foam, but I'd prefer a tougher material.

thank youuuu. ♥