NRS sells PVC material used for inflatable kayaks. They also sell glue to glue it together.

My take on air bags is that if you splinter a wood/glass hull, you risk popping the air bag too. My idea would be to make your air bag, but fill it with styrofoam peanuts so that if it got a hole it would still displace enough water to keep you on the correct side of the water (the top side). The peanuts would allow flexibility as opposed to solid foam, so that you can squeeze it through your deck hatches if they happen to be smaller than your float bag.

For making a bow/stern bag you could go to the water fountain and get one of those paper funnel-shaped cups. Unwrap the paper cup. Now enlarge that about 20 times and you have a pattern for your float bag. Just glue it up with PVC glue in a funnel shape and seal the open end to itself. You can tailor the shape as needed.

As for sizing, a gallon of water weighs about 8 lbs, so use that as a rule of thumb to decide how big you need based on the weight of your yak and gear. If you constantly carry a lot of gear, especially gear that does not float like maybe a big camera tripod, then you may want to make a slightly larger bag. I carry lots of camera gear and a tripod that could anchor a cruise ship, so I always ensure lots of floatation.

Also if you have not made a minicell foam seat yet, that will displace some water, and don't cut off any more extra foam than you have to, such as behind the seat. 3" by 20" by 6" of foam left behind the seat will displace a few pounds of water, and will make your cockpit easier to bale by a few pumps. And then there are minicell bulkheads, which even if you got hit by a speedboat and somehow lived and your kayak were splintered you would still have a nice piece of foam to float back on. (Perhaps a slightly exaggerated example, but you get the picture. )

-Malcolm