Got mine for $14.00, and they only had the one for 8+, however its either fitting large or I have the shoulder of an 8yr old. Here's a quick cameraphone pic, I took off the existing cannon, and added an arm I bought from a lair member, then the cannon made by Mr. Fett
Needs plenty more to stabalize it, but I believe this will work a treat on my own suit.
I actually bought it at my local Target and was going to ask you guys what you tough about using it as a plasma cannon. I know it quite doesnt look the same but it would save time and especially because I'm a SUPER NOB to constructing things and building the Predator suit.