First up, here is a picture of what I've done with the top of the water tank. in the absence of pictures or drawings I'm hoping I've got something which is plausible. I stuck some plastruct angle inside the tank and then tried to model some tie-bars to hold it all together. The water is clear acrylic with the bottom painted black.
Next is a little experiment on one of those things which is just so much easier to make when you have something that cuts accurately! I drew up this wagon turntable and cut it as 2 layers of 1.5mm MDF stuck onto a base of 3mm MDF. The rails are just short lengths of phosphor bronze rail stuck in place. The fact that the slots are accurate means that it is easy just to get the gauge and alignment right.
A wagon rolls on it and it turns very easily.
I'm half wondering about cutting a few of these and seeing if anyone would like to buy them off me. I'd have to modify the drawing to go from 18.83 to EM or OO but that should be straight-forward. I'm thinking it will be quite easy to drive with a servo to make it turn, I've not made any attempt to power the rails as I don't think locos would have past over it.