Right. Since I posted, I think I've narrowed it down to three - 2 hanging (here and here) and 1 floating (here). The floating one is large enough and has a lip so I wouldn't worry about the water getting up on the actual basking platform. The only reason I'm considering a floating one is for convenience - it changes with the level of the water whereas the other two require manual intervention.

Any thoughts on those three I'm currently looking at?

Also, I assume my turt will be fine a for a few days for one of these to arrive if she can't get to a basking area? She goes days without basking anyway, so I figure it would be ok.