I just bought a transparent smoke polycarbonate case from EB, and I can say that the clear (dark clear) case looks awesome on the Cobalt. It's a very well made polycarbonate case, nice and snug, and it was $14.95
Don't get a clear white one cause it makes your cobalt or crimson look like crap (in my opinion).
It does change the feeling of the backplate though, from the gritty matte feel to shiny polycarb feel, if that matters to you.