What a lovely, fun place for box turtles to hang out! I have been building an enclosure with cinder block and thought I'd need to make it two bricks high. Have you had any issues with the boxies trying to escape over the single-brick part of the enclosure? (Though maybe they wouldn't want to...
Your descriptions "solid dark olive green" and "WW2 helmet" make me think of Three-Toed box turtles I've seen! Some have yellow markings but some, especially older ones, will just be an olive color all over. And their shells are pretty high-domed compared to fully aquatic turtles. Eastern Kansas...