Although it's not in the instructions this way, I would mount all 4 through the outer and inner lids with rubber washers aand flat steel washers on both sides of the rubber washers to take up the space between the inner and outer lids. I would also use Star washers on the outside under the rack and large washers under the bolt heads inside.
Make absolutely sure the racks sets on all 4 legs and doesn't rock before you bolt it done or it will stress the lid.
Use blue loc-tite on the bolts and do not over torque them!
All this, and MAYBE you'll be one of the lucky ones that do not crack their lids.
My answer was:
http://www.pakitrak.com/