Andy, Great idea! I’m slowly making one of these in 1/29th scale myself. Below are pics of one I made years ago in ‘O’ scale for my basement layout. Being detail oriented (read that as anal about accuracy) I can tell you I measured several. They vary in size and shape. The ‘Squished’ top one is an ‘ellipsoidal’ water tank. The round tank I got up close to was called a Horton Watersphere. As for dimensions, what I can tell you is the vertical support tube should be 6 feet in diameter (that’s 2.5" in 1/29th). On my ‘O’ scale tower I used a 1.5" OD acrylic tube. For my 1/29th WT I am planning on using a 2" PVC pipe (the higher pressure schedule so the OD is almost 2.5"). On the watersphere I made in ‘O’ scale the sphere should have been 28 feet in diameter (that would have been 7" but I could only find a 6" diameter ball so that’s what I used) So your choice of a 12" ball is perfect for this type watertower. I’m making mine 12" also! The bases of watertowers vary quite a bit so whatever you do will probably be close to a prototype somewhere. My watertower is named Wilbur after my dad’s friend who had a great American Flyer layout in his basement and was the person who kept my brother and I interested in model railroading
Keep up the good work! Brian B.