Originally Posted by
GoktimusPrime
Maybe have the front half of your legs look like the cartoon model, and the back halves look like the toy. Because as you rightly said, the cartoon model's legs are quite slim and humanoid with no alt mode kibble -- it's a complete animation cheat where they just magically morph into the rear part of the cannon... and that just doesn't work IRL. The toy of course rotates the waist 180 degrees which also doesn't work for a cosplay unless you're an extremely talented contortionist. ;) So perhaps go for a leg design where it looks cartoon accurate from the front, but toy accurate from the back, so when you lie down to transform your legs have already formed the rear section of the cannon (all you have to do is keep your legs together).
<shrug> Just a suggestion. And good luck! Please post photos when you're done. ;) Are you planning on wearing this costume at a particular event?