This is a tight fit over the 2020 extrusion. If a friction fit is not enough, you can use a M4 T-nut to hold it in place.

The holes for M3 bolts holding the XBee down are designed to either be tapped or drilled out to the appropriate size after printing

FreeCAD sources are attached so you can easily alter this to fit your needs. I've used similar designs for other circuit boards: edit “Sketch” to alter the shape, and edit “Sketch001” to set up the hole pattern