I used to use two 10 ft cam straps per bay in the past to hang my cooler off. It worked fine, but the cams would inevitably loosen and rotate, which is a bit of a pain in the ass over a 7 day trip.
I finally bought a pair of the NRS adjustable cooler mounts to test them - they are AWESOME compared to the cams. They don't loosen nearly as much, and they have the 90 degree metal on the bottom that holds the cooler in the same position at all times. They make life much easier when rigging, I just leave them on my frame now rather than have to setup the cams every time. They also make the cooler/dry box much more stable to walk on as well as cake to open when you need something mid-day.
You could probably make something similar and it would work, but in the end I just shelled out the $50 and would do it again in a heartbeat.