The circumference of the paper bag is about 16" (folded bag laid flat was about 8" across). The paper bag in the 3D depiction is well within the circumference.
This is some dispute as to which jacket Oswald wore to work, so a generic jacket-collar was used (the default white color doesn't mean I think he was wearing that color of jacket).
The area visible between the right armpit and right hand would have been totally filled with the paper bag color. According to the WCR, 34.8" is the length of the wooden stock.
I was able to get the end of the barrel through the detached front band, which rests at the bottom of the bag.
I have allowed room for the trigger assembly but have not drawn it yet on my Carcano model.
The oversized numerals are as they appear in SketchUp.