I used parts from Northern Tool and the hardware store. The rack is all steel and uses 1" gas line pipe for the bars. My bars are 72" and comfortably fit two canoes.
I started by mounting L-track to the topper at all four corners by through bolting with 2" fender washers on the inside. This stuff has a load rating >1000lbs and allows me to quickly disconnect the rack when not needed.
Even if you wanted to stop here, you could use these as a tie down point and mount a canoe up top on foam blocks.
I'll have to upload a more detailed photo of the rack that I built later.