I was getting to this point with my own design. Mr_Terasaur did a nice job with the shoulders so I merged his with mine. Shaved a little plastic out of his design and added a proper DAS peg so it clicks in nice and secure now.
I put my buckle in there also. It works or you can use the one he made.
Update 6/19/19 Added a 3.1 full shoulder version.
Some people were having trouble with the tabs on the bottom staying in place while printing. I added a full shoulder version to attempt to eliminate the issue. If you print one let me know how it works out as I won't be able to test till this weekend.
I have also found using a brim on my parts even with supports seems to help this part alot.
Update 6/30/19 Added split files for full shoulder version. Now you can print them one at a time