Have you considered what I have seen a few people do, and rig the backpack and plate together such that the backplate harness becomes the backpack harness? I have always wanted to try that, but I think I'd need a sewing machine to do a good job of it.I can take a few pictures when I get home from work.
The backpack itself is just some random cheap backpack my wife had laying around. It's nothing special and not very large. Probably sized for a 15" laptop.
I do break down my bp/w for travel as it packs a lot easier with the wing and harness off. The plate slides nicely into the laptop pocket, etc.