How would you build a room on a barge for instrumentation?

I am a senior at the University of New Hampshire studying civil engineering and am working on designing a barge that will be a platform to test various styles of tidal powered turbines. What I am wondering is, how would you build a room (like a shack or shed) on top of a barge that will house instrumentation, a person or two, a computer and anything else that we would want to keep out of the elements. The barge is going to be approximately 30 feet by 60 feet and the current design is looking like two HDPE pontoons will be used. What kind of materials would people use and why? I'm trying to do some initial research here and if anyone could point me in the right direction or know of a boat builder that would have any ideas, any input is welcome. Thank you.