One point that I would like to add to the post of "How To Do It" ...
if using foam, I recommend you NOT give the surface a sealing coat of epoxy.
You will wind up with a rough surface once epoxy cures, and must re-sand/fair it back down. If factory smooth finish is still there - don't worry about sealing it. If a cut and sanded surface is there, get an old iron (clothes type - don't use the wife's!!!) Try it on some scrap, but heat it to the point where running the iron over the surface of the foam, will heat, melt and smooth the foam's surface. This seals the foam, and even less epoxy is needed.
Picked up this trick via r/c sailing, where weight containment is even more critical on 1 to 2 meter multihulls than it is on 18 footer real boats.
Another thought, is to lay up foam strips similar to a strip kayak or canoe. Cut on bandsaw, and run edges through a jig on router table adding bead and cove profiles edges. These allow for the strips to fit together, but the rounded edge also allows for the strips to take on a curve without producing the gaps which need to be filled. Strip foam construction requires additional support to prevent the strips from bowing between your cross section templates - especially if you are using 1/4 inch thick strips. What is nice about this method of foam cores, is the ability to cut the strips to thickness needed - so in the hull area, you can use maybe 3/8 to 1/2 inch, while on the flatter sides or on deck areas not subject to loads, you can go down to 1/4 inch for core. A home made hot-wire to cut/shape foam is also very useful. Lots of plans on the internet. I used stainless steel fishing leader wire for the heated wire part and a small electric train transformer for the power supply to heat up the wire. This also leaves a very nice, smooth finish that doesn't take up a lot of epoxy. If you want to get fancy, you can use untempered 1/8 inch hardboard or thin sheet aluminum to make foil shapes. Once you cut the foil shape, the can be added to the front of round cross beams to fair in and give a nice result.
