Ender 3 Frame Mount Extruder Bracket

The idea is pretty universal, therefore I've added the .STEP file for you to modify as needed.

In the slicer, I faced the extruder face down on the build plate and added supports as shown in my images so all the mounting surfaces were printed true to each other.

I like the extruder facing the front for easy maintenance.  You can also install the bracket on top of the frame if you don't want to limit your Z travel.