said by ArgMeMatey:+1, a shelf made out of tile will be too weak and look fragile. Making something from one of these materials will look and perform much better.
For something cheap and easy to clean, I might go to a place that makes countertops and find a matching remnant of granite, Corian, etc. Have it cut to size and then use silicone caulk to fix it in place.
Otherwise I'd hire an experienced tile pro and tell him/her what I want.