The upper hull is made of 4mm ply which turned out to be rather splintery. When I built this (it was the first thing I cut) I tried edge-glueing it, which wasn't strong enough. I then tried supporting it with wooden strips inside, which worked to an
extent, but it was hard to get the angles right. I was considering covering it with fibreglass, but then remembered papier mache from my childhood... I mixed up PVA wood glue with water (about 50/50) ina bucket and then added torn-up newspaper. Soaked for a few hours, you can spread it over the top to make a smooth surface, or squidge it into a sort of putty which sticks to the inside of an angle and holds it rigidly in place. After it dried I could easily stand on the tank.
Warning- I should have sanded it down before painting, as paper lines are still visible.
I put the clock in the background because it too is made of papier mache- not by me, I hasten to add, but by a Japanese artist working in London in the 1920s. Papier mache was invented by the Chinese who used it to make helmets, of all things.