The only free plans that I knew of until now are at www.microhydros.com which would have to be scaled up.
It's kind of too bad that you're going to be using an .18 as the IMPBA now has a class in the proposal stage that as few of us are building to see what we can do with them using .12 engines. Mine's really close to hitting the water now. Any hydro from old shovel-nose to the newest designs can be used as long as they're within 20-30 inches long. You are in the right neighborhood though trying to find one in the 26"-30" range.
There are some kits out there made out of plastic but wouldn't advise using those as plastic doesn't like nitro and I was also told that they won't hold up well to the vibrations.