Looks good! How stable is it? If you are still thinking about using a junk rear hatch for building on top of, I found a hatch in my area. If you could hold off until June on making a permanent hatch, I may be going to a Vibe meet in central Michigan at that time, and could bring it with. Or it could be FedEx'd. I could check the price on it if you're interested. Here's the link:
2007 TOYOTA MATRIX PARTS CAR
Regardless, you've got a good start there. Have you tried driving with what you've got on the car, just to check it out? How does the hatch open with that on?