the problem with building something like this is that it's either going to be huge in size, look awful, or both. for each system you need access to the cartridge bay or cd slot as well as any controller and memory card slots... and since each system is configured differently (i.e. top-loading or front-loading), it would be hard to pull off. plus, you'd probably want to make sure each system was vented properly to prevent overheating.
my suggestion is, make a small flexy rack. this way you can add/remove shelves, adjust shelf height on the fly, and the best part... it's CHEAP, especially if you make a small one.
here are some sites that i'm following to make a flexy of my own (although i'm using mine for a home theater rack, so it will be much bigger, stronger, and more expensive):
http://www.tnt-audio.com/clinica/flexye.html
http://my.execpc.com/~saruman/rack/nonemoreblack.html
http://mikelietz.org/etc/avrack.shtml
http://www.positive-feedback.com/Issue16/diyrack.htm
http://members.cox.net/~tonyptx/homeaudio/flexyrack/
the flexy-style rack is highly customizable, so you can tailor your rack to your needs... if i were you, i'd probably go with some 1/2" rod and 1/2" MDF... all of which (including hardware) can be found at home depot and lowes. i'd cut my MDF shelves to be just big enough to fit an xbox comfortably, and put all the switchers on the top shelf.
anyway, i'll help out more later if you're interested... my girlfriend is demanding i cook dinner. lol