Perhaps not the wisest strategy, but I'm basically designing my PFD as I go. So far I've made a vest out of 70D nylon, and soon I'm going to start work on adding the air chambers. I've got some neat ideas for the air chambers, but no valves yet. I'm hoping to get the chambers mostly made and installed and then I'll add the mouth valves when they arrive.
It's my first time doing flat felled seams and using edging, so the vest is already looking more professional than stuff I've done in the past. Too bad I didn't order quite enough edging.
The vest I've got so far is 1.55oz. I think the chambers are going to weigh about 1oz, 1oz and 1.5oz, so theoretically I'll wind up at 5oz before I add stuff like buckles and pockets. 2 buckles with webbing is an ounce and the pockets are maybe 0.5oz, so my wild guess is 6.5oz.