I've got a quick release clamp on my seat, but I'd like to replace it with something that is a little less easy to steal (like a hex wrench or something). Is it possible to buy just a hex bolt to fit my existing clamp, or should I get a new clamp with bolt?

I found one on Amazon (http://www.amazon.com/Eleven81-Alloy...eat+post+clamp), but i'm not sure what size I need. The outside of the frame where the seat post inserts measures 31.9 with a digital caliper (when the current clamp is tightly attached). Would a 31.8 mm clamp work, or do I need to go a size up?

Thanks!