Hi Gantu & Dietzer --
The side view I posted above is from the new book: "I-400: Japan's Secret Aircraft-Carrying Strike Submarine" by Henry Sakaida, Gary Nila, and Koji Takaki. This is the side view, the book also has top, front and rear views also. These drawings may not be exact, but they look better than anything I've seen before. I think they are definitely good enough to build a model from. The Seiran drawings in the book are excellent.
Unlike the i-400, a Seiran survived the war intact and has been fully restored. It's on display at the NASM Udvar-Hazy Center at Dulles Airport.
This is going to be a long-term project, and I can't even really get started yet because I have a couple of other projects to finish first. Ever since I got the new book, I've been daydreaming about this project here and there.
Mylo -- I like the trailer idea, unfortunately though I don't have any place to conveniently store such a trailer. I have a small house and yard and a garage already full of equipment and models. I'll think about it though. Hmm... I wonder if the sub would even fit in my garage at full size...
I would definitely use foam or whatever to make sure the model never sinks! I built a Robbe U-47 a few years back and ended up having to retrieve it from 20 feet of water with scuba gear!