I have built one fish house and been in many more. For the bunchs, I would make the bottom bunch foldable. That way you can put a couple holes underneath it or just have the extra space for more people. A piece of canvas stretched between a metal frame makes a very nice bunch. For insulation, the spray in foam works very well. The inside of the house looks a little like a cave, but you don’t have to put any paneling on the inside of the house. There is a plastic paneling that is in many public bathrooms. It is white and waterproof. My friend put it in his house and it adds a lot of light and helps protect the wood inside from water. It is very easy to clean if you get a flopping fish also.
I am sure you will get many more interesting ideas and I will be waiting to hear more ideas too.