: 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.

Get a large palstic garbage bag and use it to line the area you wish to fill with foam. Put the liquid foam into the bag. When it hardens you can pull out the bag, with the foam inside it. This saves you from having to wax the inside of the boat. You can cast your flotation in several adjacent bags,(front half and back half, or split the area into thirds) and just put in as many as you wish, giving you some cargo space when needed. When you do it this way you don't build up as much heat or pressure from the expanding foam.

It is cheaper to buy foam insulation panels at the home center or lumber yard. Get thes in 1 1/2 or 2 inch thickness if you can. You can cut these to size with a sharp knife, or a hot wire. Start with a piece standing on edge that lines up with the center of the boat. Shape that to fit the profile of your boat and shove it into the area you want to fill. Then make two identical parts that are a bit smaller, one for the left and one for the right side of that first piece. If you are using 2" material you'll now have a plug that is 6" wide. Keep adding progressively smaller pairs of foam panels to the left and right until you are happy with the result. Each pair of panels adds 4" of width, so for a 20 inch area you'll cut the center and 9 pairs of side pieces. You'll save a bit of time by cutting the two pieces at one time. They don't need to be a perfect fit, in fact, if you keep the edges aquare,instead of forming them to fit the curature of the boat, they'll fit a bit loosely and they won't trap moisture.
If you want to make this into one big unit, use a construction adhesive, like (R) liquid nails to glue the foam pieces to each other (not to the boat)
Check that your adhesive is compatible with your foam.

Before adding foam of any type, check that your hatches will be wide enough to remove a large block of foam whnn your paddling is finished so that the boat can drain and stay dry and mold free.

PGJ