Sorry to join the party late - I use a Rhode Gear Truck Shuttle in my R51 Pathy. It works great, but I don't think it is made anymore. I bought it cheap about 5 years ago w/ 4 mounts for less than similar mounts alone would have cost elsewhere.
Basically extruded aluminum w/ 2-4 fork mounts that bolt in at any of several angles. The whole thing lays flat toward the back of the truck, back wheel goes in first. With portions of the 2nd row folded flat, 1 or 2 bikes fit great, 3 should work, 4 would be pretty tough. One caveat - I'm 5'8" and have slightly short legs for my height. My bikes fit w/ a little room to spare. Bikes for taller friends need to have the seatpost removed or lowered to clear the roof.
You could basically replicate the thing with a single piece of dimensional lumber, $10-20 fork mounts, and some type of non-skid stuff on the bottom.
PM me if you need more information.