This the the brass catcher Ivan has been using.

The STLs are oriented and scaled correctly.

I have been using some M3 screws and 1/4 - 28 rod to assemble the catcher.  The screws are for the rail mount, the rod to attach the body to the mounts.

I drilled the mounts out to .250, and hand threaded the rod into the body of the catcher.  It has held up for about 1000 round now.

I printed in ABS, it should work in PLA.

You will need to make a bag.  I'll leave creation and affixing the bag to you.  If you use fabric, consider using fireproofing spray on it, just in case.

I used some scrap cotton cloth and hand sewed it together to form a pouch.  I then drilled four holes in the fround lip of the catcher to sew the pouch on to.  You could just use binder clips or whatever is handy.