A few years back I had a Grumman and a beater Toyota PU w/ a cap. I bolted some light angle iron uprights to the rear corners of the cap and spanned them w/ another piece of angle bolted on. Over the cab I used a basic gutter cheapie rack.
The foam blocks are an excellent way to go and can be modified w/ an electric carving knife ( cuts even better than turkey). I used a set of them when I was freighting the Grumman on an old full sized van.
I now have a light weight canoe, and will be using Thule ( expensive but EXCELLENT) racks on my Subaru wagon. I still use the foam blocks. The ones I got from Campmor have a groove in the long axis for the rack, and on the other side they have a groove for the gunnels running at right angles to the first groove. Works out nice, and there is no marking up of the gunnels by the rack.
AgMD