Unfortunately, I think you're right. Turtles are 'cute' to most people while they're small, but lose the attraction as they grow. The sad reality of it is that 4" turtles are the most sold by pet stores and so they want to sell items that fit a 4" turtle.
However, on the DIY route:
I think a 6 quart oil drain pan would be a perfect starting point. A hot knife or Dremel tool should make short work of cutting opening(s), then you can decorate with rock, plants, sand, etc to fit your decor.
http://www.coxhardware.com/p-3011-round ... quart.aspx