I built one twenty years ago that has worked ever since, as follows: 1= Excavate 1 cu. yd (3X3X3). If you are going to plant on top, make it 4 ft. deep. 2- Line the sides and bottom with styrofoam insulation, which you can buy at home depot and in 4X8 sheets and cut to fit with a knife. Make the sides only 4 ft. high if you paln to cover the drywell and plant on top. 3- Fill with coarse gravel, crushed brick - anything which will keep it from caving in. 4- Cut a hole near the top to recive your pipe. I used perforated PVC because I wanted to take away both the roof and ground water. 5- Cover the gravel with sheets of styrofoam. 6- Backfill the last 12" with topsoil if you are planting and that's it. Good luck.