Powermaster Optimus Prime, small robot. Just sit down, fold up your arms in front of you and tuck down your head.

 

Check out anemis' costumes at marksprojects.com.

He made a great sorta-Sunstreaker that transformed.

 

Energon Ironhide just sits down with his legs straight forward and his hands behind his back.

 

Almost any cartoon/toy design can be emulated with enough time and effort. Even the ones which require waist-spinning, backwards knees and whatnot. Some of them are not really suitable for the "5 minutes with cardboard and markers" approach, though.

I was working out a Classics Optimus/Magnus costume scheme just the other day. Yes, it is possible to spin a costume at underarm height, with sufficient prep time  

(Although I'll admit I'm still having trouble trying to figure out a good G1 Megs transformation that doesn't stuff the wearer's spleen up their own nostril. Omega Supreme was easier, and that involves ripping arms and legs off.)