My nephew wanted to be Bumblebee from the Transformers for Halloween so my mom made him this costume...
(blacked out his face cause my bro/sis-in-law never got back to me to let me know if I could post the pics or not)
We bought black sweatpants and sweatshirt as the base, then spraypainted a bike helmet, and finished up with a lot of spraypainted foam hotglued or velcroed into place! To do the Transformers symbol my mom created a stencil then painted it on with craftpaint.
Used LED lights in the helmet and Transformers symbol