Cap for Plastic Shelves

I have a couple of these plastic shelves from Amazon (see picture of shelf and link below to amazon posting). The shelves are held together with plastic pipes. On the top shelf, they have this nice opening where things can fall into it all the way to the bottom shelf inside the pipe. (It has eaten many items.). These caps cover the top hole to prevent this from happening. 
 

This is the style I have, Amazon.com: Gracious Living 4 Shelf Fixed Height Ventilated Medium Duty Storage Unit 14 x 32 x 54.5" Organizer System for Home, Garage, Basement, & Laundry, Black : Home & Kitchen 
It's possible it will fit other brands etc… 

The cap fit's 35.5 mm holes