I have both ortleibs and axioms. The axiom mounting system is pretty simple and I think would work well for a DIY project. They use a simple bungie hook system below and metal hooks on top with a twisting tapered block to lock it in place. I also think you could get the parts pretty cheap from Axiom.

Or just get the locking blocks and top hooks and the rest of the stuff could be found or made with stuff from your local hardware store.

These bags ride very well loaded. Good luck in your project.