Hey! I used some thin foam, chopped it into squares, wrapped it with a fabric that matched the game, and glued the piece together. Created the pattern of the vest fairly well and took very little time. Plus helped cushion the ab piece.
I didn't do the seams around the individual pieces nor the buckle at the end. If you're wanting to do that, I've seen people use tape or sew duck cloth or something similar to the edges, as well as glue/hammer in a slot for the strap.