Winch plate question on aluminum bumper

I have a used Rockhard aluminum bumper that did not come with all the hardware. Reading the installation instructions, the hardware included 2 steel plates to reinforce the winch plate (already 1/4”)

I am thinking I can just create something from Home Depot steel bar stock, but am not sure how thick it should be. Choices are 1/8” or 1/4”. I want strength, but dont want to go so thick that the winch bolts don’t fully engage.