You want to lower the water level in the tank some to accommodate a basking area for this turtle to dry out and bask. Perhaps lower the water level 35% from the top at minimum. It needs a reptile bulb at least on one side on top of UV. You may try a big piece of cork driftwood, but a big piece would be needed to support the turtle's weight. Soak the wood before you put it in. Get a nice size piece so it lasts the turtle a while and totally emerge the turtle from the water. Don't let the water or the tank temperature get too hot.