I think they are usually called "strongback". You may want to consider making a nice one out of plywood. Nick is the only designer I know of that utilizes an internal strongback. I have built the Guillemot L with a plywood internal strongback and I am sure it will be usable for many more boats, but there are lots of good designs out there that do not utilize the internal strongback. If you are planning on doing it for a business you may also want to consider steel. "Unistrut" is a company that manufactures square/rectangular steel tubing.