You might check out the TruCut Rotary Cutter from the Grace Co. I bought one and I really like it. The Rotary Cutter attaches to the side of the ruler to hold it in place.
It is a little pricey. I bought both the 6 x 24 ruler and the rotary cutter, and the ruler grips as a package deal at Tuesday Mornings for the price of just the rotary cutter.
www.graceframe.com/site/rotarycutting