Thank-you labrat and Jeff for your knowledgable comments. From reading previous postings from Jeff and others, I was aware of the safety reasons for displacing as much space as possible with floatation inside the kayak. However, both of you here, have convinced me that a proper float bag is worth buying. Also, I figure that by time I have made something comparable, I have spent more time and money than necessary.
I am still confused however, as to just how large of a float-bag is practical. Here's what I mean:
-Skinboats.org sells Greenland Float Bags that are 23" wide x 57" long. One of these bags in each end of the kayak would certainly fill most of the hull except for the cockpit. If you want stowage, they have a combination float-bag which has a compartment for your gear. I imagine though, that building a hatch in your kayak would be pointless because access to your gear could only be from the front of the bag, through the cockpit.
(This sounds inconveinient to me)
-MEC.com sell float bags that are about 30" long. These would tuck up nicely in the bow and stern and leave room for your gear. But, they would not displace much water in the event of a capsize.
(This sounds like a possible comprimise)
I'm brand new at this of course, so I have no idea what kayakers prefer. Please chime in everyone...