This cover slides in from the front (assuming you mount the unit with screws on a vertical surface), covering the USB plugs and leaving room for the switch. There is a small dot on the side to engage the plugs' ground hole to lock it in place.
Here's the model of power bar: https://www.amazon.ca/gp/product/B09CQ3L755/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1
Print face-down as in the STL.
The author marked this model as their own original creation.