It wasn't that hard to make- two pieces of wood screwed together, with corner supports on the inside for added suport. I cut holes in the wood for the straps that would hold it to the carseat. Then I added an axle and wheels.
The finished product wasn't very pretty, so I covered it all with some denim fabric (sewn and hot glued), then added the straps.
The car seat rests on the small ledge, and is held to the travelmate by two sets of straps- one goes through the LATCH straps that hook into the car seat when not in use, and the other follows the path of a regular seatbelt (under the armrests and around to the back). Once strapped in, the car seat isn't going anywhere.