Frank,
I like the comfort of knowing I have flotation. The intent is to keep the boat floating level even when swamped. I know when I built the broadbill I used 2lb density but I can't recall the quantity off top of my head. I'll check the Devlin plans when I get home and let you know. I would guess that the Broadbill is of similiar size to your BBSB. If possible, I would put some in the bow and two locations in the stern. I know with some of the kayaks I've owned and the Momarsh FB, it looks like they've been stood on end and poured. Once that's set it's been flipped and the other locations poured. So in other words, I don't think you need to have a special compartment if your boat was built without it. I would also recommend getting the closed cell so it doesn't soak up water.