There are many ways. And you don't necessarily need a divider for the land/ water.
2 ways that might work for a tank that size:
Egg crate is easy to use. It is the plastic (styrene, actually) stuff that they use below lights in hung ceilings. They sell it at Home Depot, etc, in the ceiling section. You can use it to make a false bottom, and hold it out of the water with supports made of egg crate. Disguise the edges with stuff (wood, plantings, etc.) so it does not show.
Another way is clay. This is a method that has really gained in the past few years. Essentially, make a firm clay, mold your land, and let it dry some, and once the biofilm sets up it is pretty solid.
What kind of frogs do you have?