In addition to tactical bags, which might be very good, try online for sites like Miners (www.minerox.com), Ben Medows (www.benmedows.com), Forestry Suppliers (www.forestry-suppliers.com), or others that have bags for Geologists, wildland fire crews, etc. Roy Gfeller, True North, Stat Packs and severak others make packs for carrying heavy, small loads (like rocks or fire tools). Be prepared for the cost though. My experience (rocks and equipment mostly) is that zippers are not the best choice. Not too many top opening packs left, so you might use one where only the upper part is zippered. Not a full opening so that the zipper does not carry much of the load.

Respectfully,

Jerry