I started customizing Xbox 360s few years ago, and made tutorial video:
artistpavel.com: Video: Painting Xbox 360 Faceplate In 3 Steps
back then I used automotive primer for a base coat available at Curry's Art Store, but later I discovered Krylon products, their stuff just sticks to any surface. You just have to make sure not to overspray it to avoid paint bleeding and build-up, it can't be removed.
this topic made me think if should offer custom art services for PC users, since I have all the experience, but for some reason was limited to Xbox community. If anyone wants to give it a try, I'm based in Toronto. The way I think it can work, you can send me the removable side panel and I will paint design on it, like I did with 360 faceplates:
Xbox 360 Faceplates - a set on Flickr
for example if I was to paint HWC rig it would have a beaver and red and gray decorative elements.