How should i set up my cage?

I caught a wild bullfrog at my grandparents pond and the water is murky and very warm. there are a lot of other big bullfrogs there and the pond is about 200 feet by 50 feet. the deeper end is about 12 feet deep and the shallow end is about 3 feet deep.it is rarley mowed so there are grasses over 6 feet tall in some places but in most places the grass is 2-3 feet tall. the bank is pretty steep and goes down about a foot at a 45 degree angel. the grass doesnt go to the edge of the pond but there are some plants and shorter grass around the edge. I am getting a 20 gallon tank for it, 2 feet by 1 foot and about 1.5 feet tall. how should I set the cage up with water and plants and just about everything? the frog is about 3.5 inches long without legs.