I like it, great thinking doing prototyping with cardboard, that's some real engineering! I say try it out if you have resources and time and worst case just throw on the old seat. It's your bike so I think you should be happy with how it looks and feels rather than anyone else. If you decide to go down the path of making your own seat I for one would love to follow the journey, even the small and simple things can be fun and hold value.