If you can't mount PVC pipes to your cross bars, then I would add 2 ea. 4" x 4" 's cross wise on the trailer separate maybe 5 feet apart....then mount 4 ea. 3" OD x 6' long PVC pipes length wise that are screwed into the 4 x 4's to support the Outbacks.....the PVC pipes cradle the Outback and will "sag to take the hull shape".
If you turn over the Outback and lay the PVC pipes into the hull indentations, you can get the correct pipe separation dimensions for mounting to the 4 x 4's.
Then, look into reducing the air pressure of your tires.
I own a Hobie Trailex aluminum trailer that has 4.8 x 8 tires, load rating B and max load is 590 pounds with 60 psi cold inflation......HOWEVER, the trailer tire pressure, specified by Trailex is only 15 psi maximum........the gross weight of my trailer and spare tire with my PA-14 is about 310 pounds.
I have used my trailer at highway speeds for several hundred miles and the tires show very little wear and the trailer bounce in minimal.
Hope this helps....here is a photo of my trailer set up.