go down to your local conveniece store, look around back where they load supplies, and look for the pepsi or coca-cola heavy plastic trays, black, blue or red
there about 15" by 24" by 4" or 6 " deep
made outta the same stuff as milk crates, then i either use heavy ty-raps for short use or u-bolts for long term and mount it to my grab bar
i have a few, one holds 2 small 6 pack coolers side by side, the other i made a few cuts and slots too, that hold my bow and other hunting stuff, and just bungee my stuff down
i have been using these for years and never had one break, through countless crashes,
they're pretty durable, and the best part - free