Making pockets for the air chambers instead of sewing them on wouldn't take that much fabric. Maybe an ounce, plus whatever other weight you add in terms of zippers or snaps...so 2-3oz if you had a lot of hardware.
However, making the air chambers is by far the hardest and time consuming part of this (getting all the measurements draw on the material, cutting it, adding baffles and heat sealing etc). So if you actually did this you'd probably want to use the same chamber for all 3-4 spots instead of unique chambers, so you'd only need to carry one spare. Creating 3 unique spares would add a lot of work.
The biggest challenge with a PFD seems to be getting enough inflation on the front of your torso. It's easy to add tons of float to the back because it's out of the way. With the front though, you need to get quite a bit of float out of a relatively small area. I couldn't make the front air chambers any lower, because when you're sitting down you don't want them hitting your legs and being shoved up. I could have made them a little taller, but it starts to get interfere with the movement of your arms and the vest is narrowing quite a bit so there's not that much room anyways.
My point is just that if you didn't use baffles and just mostly used 'sewn thru' construction (but heat sealed) then it would wrap around your body better but you'd have a hard time getting enough air volume. Even as it is, I'm nervous about how much I have on the front. I have about 8 lbs of float on the front and 9.7 on the back. It might float alright, or I'm might need to partially deflate the back chamber to float on a nice lay back position.
One approach that would work well would be to NOT have the closure up the front, as it takes up valuable space. If you made a vest that was a pull over and then attached at the sides, you'd have an intact front area to work with. With this, you might be able to use a 'sewn thru' construction technique and still get enough air volume.
The front pads on this are 1.5" thick. Originally I made chambers that were 2.5" thick but it was too bulky. I went to 1.5" thick and maybe them a little larger. It actually feels quite good. It's fine if the chambers are firmly inflated, and if you back it off 10% then you don't even notice they're there.