For an idea of shape, pictures of the casings can be found here
http://www.wgcshop.com/pcart/shopper.php?cat=Gearboxs though there are plenty of suppliers in Hong Kong and the US.
I'd probably go with a version that is used in all the decent M249's that are available (and happens to be one I have spare from upgrading my own M249) It also helps that it's basically a single unit with a regular shape with a simple micro switch for activation that can be actuated in a number of ways.
Dimensions are 220mm long x 65mm high x 35mm thick
http://www.starairsoft.com/product_detail.php?productid=64
Dreg is not entirely accurate with his comment on parts. The vast majority of internal parts are fully interchangeable. As with any technical hobby there is a huge range of aftermarket parts for upgrades from a range of manufacturers. Of course the range is rather dizzying. So far as the shonky cheap plastic Chinese shite goes, that's another matter.
Mostly I was thinking out load after having a brainwave from seeing Link's picture of the templates all laid out. Looking at the AR it strikes me that it has a lot of potential internal space in there that would lend itself well to an airsoft gun.
I'd pop a large battery in the foregrip section and use a barrel and breach unit from an M4 (really simple and freely available.) A removable clip strikes me as rather a fiddly prospect, I'd investigate ripping the guts out of a powered magazine and having a reservoir in the top unit.
Once you get a hang of how an AEG works, it really isn't rocket science; however it is fiddly and can have some esoteric issues. As a first project it might not be the best of ideas (lord knows I've experienced enough expense and fury messing around with the kit I have.) However in the design of the AR there is potential to make a truly godly BB hose of a gun.
The biggest problem I could foresee would be assembling the finished item in such a way that would allow access to the internals for maintenance and repair.
I have to admit safety issues at a con wouldn't be a problem. Myself and friends use airsoft guns as larp props, just have it unloaded and the battery removed or at least unplugged.