You could use longitudinal storage bulkheads like John Monfoe built into his kayak.

Pour-in-place foam would work well, too. You might consider waxing the inside of the hull before pouring the foam, then you could have a removable float--tie it in when needed, or remove when you need the storage space.

How about some really cool hard-shell plywood floats that exactly fit the space, and can be used as luggage like the hard "saddle bags" on a Honda Gold Wing!

All your ideas are good ones--try the best and let us know how it works!

Shawn

: I'm contemplating building a Pygmy Triple, and have read enough tandem rescue
: horror stories to be concerned about the amount of water it could take on.
: Sea socks are one option, but it seems that the space outboard of the
: cockpits could be used for fixed flotation. I've used carved 2"
: sheathing foam to fill uneeded space in plastic boats, but was wondering
: if anyone had tried pour-in-place foam, longitudinal bulkheads, or other
: methods.