Most of them are done at the factory, but I know a few people who have built their own.
Do you have a little shelf between the two opposing couches? You can cut some THICK starboard (perhaps two layers) to make the base of the bed, and have it rest on this little shelf. Then have an upholstery shop make you the center cushion.
If you do not have the little shelf molded in, you can make a little ledge out of strips of starboard screwed into the fronts of the couches. This will also support the bed provided you set it up right.