We needed to adapt a 30 gallon 'breeder' tank for use with a pair of African Sideneck Mud turtles. The picture above shows the equipment, beyond the tank itself, that I use. I use 3/8th's inch plastic for the suspended land area. If the land area is very small, a thinner plastic can be used, but I find that if the plastic is too thin, it will bend too much when filled with dirt, leading to leaks.
For sealing, I use 100% silicon rubber. A small tube can be used if you are only going to convert one tank; I buy it in large tubes and use a caulking gun to dispense it.
The funny-shaped knife is a plastic sheet cutter. To use it, draw a guideline on the plastic where you want to cut it. The hooked end of the knife blade is sharp; use it to scribe a line over the guideline. Repeated scribings will slowly gouge a line in the plastic. Eventually the line will be deep enough to allow the plastic to snap along the line. It is also possible to saw the plastic, but be careful not to saw too long, as the heat generated may affect the way the plastic cuts.
Cut two pieces of plastic if you are going to put the land area at one end, and three pieces if the land area is to be in the middle. Measure the inside width of the tank. For our 30 gallon tank, the width of the tank is 18 inches if measured on the outside, but each pane of glass is 1/4 inch wide, so the width of the land area is actually 17 1/2 inches. The length of the land area can be whatever you desire - for this tank, it will be 10 inches. Finally, the corners on along what will be the back edge of the land area need to be removed, to accomodate the aquarium sealant that runs up the joint at each corner. A hacksaw does the job nicely.
The second (and third, if necessary) piece will be the vertical dam. It should be the same width (17 1/2 inches) and as long as you want the land area to be deep. In this case, I'll cut it to 4 1/2 inches. The two pieces are shown below:
