EUREKA!!! I spent all night rummaging around the house, looking for ideas. Around 1:30am, I found this in my mother's car:
http://www.amazon.com/United-Solutions-BA0071-Storage-Utility/dp/B008BU6MQM She bought it yesterday at a craft store.
The bottom is much narrower than any other bucket I could find, and the top is about 13" in diameter - just large enough for my turtle to bask on comfortably. As an added bonus - the lid is molded with indentations and slopes down towards the rim - if I keep the water level just right, she should be able to climb up there without a ramp. It's made by Rubbermaid's parent company - I'm going to guess/hope it's aquarium safe, since the description mentions pet food, and it appears to be made of the same material as the Rubbermaid/ Sterilite bin I'm using as temporary turtle housing.
It is pretty flimsy (walls are super thin) and she’ll still lose swimming space, but should work if I position it next to the bulkhead.
STEVE AND JEREMIAH, I CANNOT THANK YOU ENOUGH FOR ALL OF YOUR HELP!!! And everyone else who replied too! (I may not be able to help with turtle advise, but if anyone has pet birds - I'm your girl!)
I saved this thread, and all of your diagrams, so I can work on a more permanent basking dock in the future. I’ve been so frustrated with this thing; I felt like I completely failed my turtle. Which was tough because she’s been my baby for 21 years. Your ideas and encouragement really saved me from giving up and scrapping the stock tank all together - thank you, thank you, thank you!