If you are not set on a hard bin, you could consider one of these waterproof (resistant) car top carriers for much less $$. It should fit on your rear seat deck and, when not in use, fold into a storage bag which you should be able to find a place to store in the cart for random use. I have one for my car that I've used for years, different brand, and never hand any water infiltration.
Amazon.com

Car trunk organizer
 

QUOTE=spinner1001;2096091]I have been looking for a portable cargo box to sit atop of the fold down rear seats of my golf cart to haul groceries, etc. I don’t play golf and don’t have a place for the mesh bag that attaches to a golf bag holder at the rear. I would like the cargo box to be portable and light enough to take on and off easily. Does anyone have any suggestions?[/QUOTE]
Amazon-Car Trunk Organizer,$21.99

PVC cargo carrier
 

Here’s a cargo carrier that I made from PVC pipe and connectors. I used Velcro hold down straps so it is easy to remove the carrier when I need access to the rear seat…

Lowel’s

Thanks for the suggestions.

I ordered a Cargo Caddy box Cargo Caddie Box With Straps (Universal Fit) | BuggiesUnlimited.com and it’s good in the rear seats when they are folded down. It’s a bigger than what I need. But it’s huge to store in the garage when not is use.

I am going to order this trunk organizer that holds 50 pounds Amazon.com and is collapsible when not in use. If this works, I’ll return the cargo box.