I think something along this design would be best for your design trailer and has a good pivot point.
If you don't want to use the axle mount and still want to use the seat-stay's since you seem to be making it yourself, I'd take some round tubing 6" or so long for stability the size of your seat stay cut it in half line it with some old tire tube to give some protection to the frame paint. I think I'd also add something to the chain stay to make it triangular for strength.
Alter the design on the Bob design to fit the new mounts make some type of clamping system for the half tube to the stays hose clamps would do but I'd prefer something a little more solid.
And good luck on your trip and for something like this I'd want it on good and tight.