At the local range the tables are made from concrete and look like a fat "T" I would set 3 treated 4x4 posts at the height that comes up to about waist level since you will be adding 4" above that by cutting out a 3/4" treated plywood "T" and then framing it with 2x4 sides,I would make the front about 3 feet wide and 12" deep with the leg of the "T" extendind back about 42",once you nave the Plywood T cutout you can figur where to set the 3 posts to support it,then once you have it framed up mix some Quikcrete fill it and use a straight board to screte it level and then smooth with a trowel

