Crossed posts with Pitlamp, but will post anyway as I've written it. Looks like we are both advocating the same method...
To be honest Josh, I think it will probably be cheaper for you to start with a cheap/free second hand bcd, rather than starting from scratch.
If you get a horseshoe shaped bcd, you can take the bladder and just cut off the side without the inflator fitting, roll up and stick the cut end with a bit of Aquasure. Then make yourself a canvas bag the right size and stuff the bladder in. Job done.
If you want to start from scratch, as above, but you'll need to find an inflator fitting and you'll need to buy a dry bag for the bladder. Unless you have the bits already, I'd be sure it would actually work out more expensive than bastardising a cheap bed.