Thanks for the comments! I sort of put the platforms together myself, using disposable plastic boxes at work that would otherwise be trashed. I attached suction cups to the lid of a box that's about 5"x8". Then attached another flat piece of plastic to it at an angle (by wedging something like a rod between it and the platform), serving as the ramp.
I've also seen pictures where people invert a shallow plastic tray (which you can get at a dollar store, for organizing little things in the house), attach some strings to it and suspend it on the water surface. Or you can use tupperware lids. I think that should work pretty well too! The good thing about these DIY platforms is that you can always custom make them to accommodate your turtles. Hope that helps! 