This is a very simple design that bolts to the Ender 3/Pro at the end of the 4040 extrusion using the board housing bolts. This creates a platform that is 90mmx90mm that stands 5mm taller than the extrusion.

This base is out of the way of the build plate so it could be used as a small platform for whatever you may need it for. I personally use it for my phone to get a time-lapse that looks along the build plate.