I think you could make it a bit narrower and i would pull air in through the rad and make sure that air is hitting that chipset heatsink, it gets hot.

It may not look the best but plywood (or any thin wood) makes a very decent chassis. I got lucky and found an entertainment center on the curb and the arima board mounts on one of the shelves perfectly. i'll try to take a shot of it tomorrow so you can see what i'm saying and who knows maybe give you an idea or two