Thanks guys!
It was a lot of trial and error with poster board. I wanted to make it as small as I could, but still protect as much as I could. I made two guards, one out of 2" and one out of 1 1/2". I went with the 1 1/2", the 2" was bulky looking. The whole bumper is 3/16". I can lift the bumper myself, it's not too heavy.