Looking at the last page in the manual for a Mini Liberty it shows their is a bracket that the magnets mount too. http://www.whelen.com/install/147/14799.pdf
You probably could get away with just bolting the magnets into the slide rail frame but will need several washers or spacers for the magnets so they aren't slammed clear down onto the frame. Otherwise the power cable will interfere with the magnets and not allow them to make full contact with the roof or metal surface that you're mounting the light bar too.