The tray is made of 1/4" plywood I had laying around. I used 1/4" spruce srips to make a frame to keep the two batteries from sliding around. I then added a coat of epoxy to seal it. I added 3/4" soft foam where the stater will lay to keep it from moving around. I also added a small piece of foam between the two batteries to keep them from rubing together. I cut a slot in the base of the tray to allow the velcro strap to come through.
