I made a small sub frame to hold my belts and mount them centrally. I wanted to make it IVA compliant, even though I didn't need one as I took my sons out in the car! Here's some pics of how I did it on a Spyder in case they can provide inspiration?
I like the way you have introduced stand off on the top bar to raise the hight of the shoulder straps.
It apears there is no need to trianglate the point where they attach in the middle.
I presume this is because the square frame has enough strength and that if the forces are strong enough
to bend the frame you internal organs would not survive anyway.
I was getting carried away with lots of cross braces compromising my luggage space.
when a couple of piece of flat bar from the floor to the anchor point will survice to mount the reel.