You've got a basking area, right?
You could look into a large floating plant.
I know a lot of people use this one (myself included!)
http://www.petco.com/product/108128/Exo ... Plant.aspx
It's a really good price (shouldn't be more than $10 for the large), and it's huge! I used to suction them down to the bottom, but a determined RES can pop it up in under half an hour. Now I just let them float freely around the tank. Both of my guys rest on it (though Frankie can sink it if she goes on top of it), and I've seen Ralph sleeping in/on his.
Only problem is that the leaves can be bitten off. I scooped another leaf (about the 3rd) from Ralph's tank the other day. Frankie pulled a couple of the branches right off, but they just pop back on.