Need to countersink holes in CF, which bit do I need?

[simplechamp]

I need to drill 4 (or possibly 8) holes in the bottom of a CF chassis for mounting lipo trays. The screws are M3, and I already have a 3mm pure carbide bit for drilling the holes, but I also need a bit to countersink the holes.

Can anyone suggest the bit I should use? I would like to find something cheap, even if that means tossing it out after I'm done. I know the CF dulls tools fast.