So the plan was/is, to have the doors of the water tank open under the fuselage, these will be quite large so inside will be visible. It will have a cargo floor and most of the tank will be under it, but also four extra tanks above the floor which would be removable for when the aircraft wasn't being used as a water tanker and reverts to hauling cargo. I have a cabin floor I was planning to use and I had bought quite a few packet of styrene strip of various section, angles, I-Beams etc so I could detail by making structrural joists and beams under the floor in the tanks area. One of the reasons the project went quite was about the floor, I had two/three other projects in mind which also need floors so I was trying to figure out how I could replicate it. But recently Carl has helped me out with another cabin floor, which frees up the one I had planned to use here.
Then, just a few weeks ago while at the hobby shop getting supplies, I found they have expanded their range of styrene and ABS sections, they have another rack of Evergreen styrene and the Plastistrut items have almost tripled. I found some items which will greatly simplify the details.
Like these sections below, this is what I was going to do with the angles and beams (cut to make T-bar)