For my IKEA RÅSKOG trolley which I use for scale model tools I made the connector to connect two plywood panels at the sides.
For my needs I printed two connectors per side in simple PLA by connecting a 10mm thick plywood panel. Even three or more can be printed, depending on the weight that needs to be supported. Three wood screws per connector are sufficient to connect the panel. Be careful to position at least one of the connectors resting it on the shelf (see photo) otherwise the side panel could slide down over time.
I also designed a handle to attach to the panel to make moving the trolley easier. You can find it here: https://www.printables.com/it/model/353269-panel-handle-parametric
The author marked this model as their own original creation.