I didn't make those components, I just borrowed them from the Sketchup Components Collection so don't give me credit for those. All I made was the frame of the case and put those in place to make the design more understandable. What was nice too was that all those components were perfectly scaled for me already, so I could base the size of the model around the components the real-world build will hold.
You could add a rail for the edges of the cards to sit on (don't bother with screw holes it'd just be for some support.)
Nice. As MonkeyNutZ said, I agree that you should add in a rail so that the cards don't slant in anyway as your picture has shown. Also, is the motherboard simply placed on the wood or are you also using standoff points so that it doesn't contact the wood?
Im working on a card support, the angle aluminum that I was planning on using doesn't fit there, so I need to find some sort of metal bar I can use to screw the cards into. I think its best to screw them in because pushing in connectors can still cause them to loosen as I've learned with normal PC cases. The board is on a neoprene foam mat, as is the bottom deck, to prevent things from sliding around.
