No plans but:
Get a hunk of seatbelt webbing, four tri-glides and two d-rings. Loop the seatbelt through both tri-glides and a D-Ring on the way up, through the attachment point, and then back down through both tri-glides and the D-Ring again. Repeat on the other side, but without going through the attachment point - that's the loop you attach when you put your BC on.
I hope that makes sense. If you need pictures, let me know.
The reason you need two tri-glides per side is only because seatbelt webbing is thin, and will move around on you unless it's 'double binded.' If you have thicker webbing, your mileage may vary. The reason I use seatbelt webbing is simply comfort. And just in case anyone blinks, my DIR-Fundamentals instructor took a look but didn't say anything.
(Tri-glides look like weightbelt keepers, but they're stainless steel... although if you were so inclined, I suppose plastic weightbelt keepers would work as well... I just don't have anything plastic on my gear.)