My shoulder harness mod is based on a fixed bracket attached to the bulkhead behind and above both the pilot and passenger seats. There are two screws there 7/8 inches apart. There are 5 cherry rivets in addition. This bracket is fixed to a seat belt harness that then positively attaches to the seat belt---could be left undone if so desired. It is a good solution and is much cheaper than my 2 1/2 years of looking for a good way to install some type of additional restraint system. I believe that it also looks good. I suppose that you would be at the mercy of your local FSDO's approval for your plane.
My mechanic would be willing to fabricate the bracket and get the belts webbed for you if you wanted. He thought that $300 would be a fair price. I believe it to be much more than fair when the nearest solution was 5 times that price. Let me know if you want his name/number. I hope that this helps.
My mechanic's name is Paul Stuber. He is in the Kansas City area at Roosterville Airport. His phone number is 816-781-7194.
Here's the 337 image front and back.
The text of the back side of the 337 reeads as follows:
Installation of two front seat shoulder harness as illustrated in figure 9.3 of AC43.1B-OA
Upper attach point is .125 2024 T3 aluminum fabricated bracket attached at station 107 at the upper edge of window (side). Attachment is with two screws (existing) AN526-832-10 and Four CherryMax Rivets.
Aluminum brackets where sent to Safety, LTD. And shoulder harness were attached to brackets.
Installed Safety, LTD. Model 1209 seat belts on L/H and R/H forward seats.
Weight and balance records were supplemented to show installation.