You're right not to put a heavy rock in the tank - or one that can fall over. Not only could it break the glass, it could also fall on the turtle!
I think in your earlier post you said you are planning to get a much bigger setup for your turtles - they are really squeezed in a 10 gallon tank. I suggested that you look into a big rubbermaid tub or a stock tank. You can get a very big one for a lot less expense than a glass aquarium. At the same time you could do something about a basking platform.
There are a couple of good basking platforms available for sale on the internet. Alternatively, you can make your own. I believe you said you are from the Dominican Republic - I don't know if your stores stock the same items as we can find here but you can probably find substitutes if you need to.
Here's a link on platforms you can buy. The first and last ones pictured are the best for your size turtles.
http://www.redearslider.com/basking_platform.html
Here's a link on platforms that you can make:
http://www.redearslider.com/custom_platforms.html