John,

Take a look at Harbor Freight. I found 2 sets right off and I would bet the cost is a lot lower than from a MC.

http://www.harborfreight.com/3-in-1-hex-bit-countersink-tool-68535.html
or
http://www.harborfreight.com/countersink-and-deburring-tool-set-66996.html

Seems like these would work great for fiberglass work. Using a normal tip might tend to "bight" and go too deep.

I have used a lot of HF tools aboard during our cruising. I always considered them as temp, but they did seem to hold up well for me.

Greg