I have a Topeak rack and a bag, but have thought that a plastic ammo box would actually be exactly what I want on the rack. Always waterproof, plenty roomy and I can bolt a big taillight to it and it won't sag and change where it points. Also, Topeak bags are $50 to $100, ammo boxes are $5. There were a couple other sled designs online but none of them were perfect. Mainly they didn't have the little ramp that helps get things on the slide easily. Also I couldn't find anyone that had replicated the latch to hold things in place. I'm very happy with how this came out.