Lay four 1 1/2-by-3 1/2-by-41-inch boards on the ground with their largest surfaces touching the ground. Form the boards into a 48-inch square. Drill two holes through each board and into the end of the board which butts into it.
Drill a 6-inch screw into each hole to connect the boards. Repeat to build another frame with four additional 1 1/2-by-3 1/2-by-41-inch boards.
Have an assistant hold these two frames upright so they're resting on a long, thin edge. Set them parallel to each other about a foot and a half apart. Position a 1 1/2-by-3 1/2-by-15-inch board in between the frames in each of the corners, with its widest side facing sideways. Drill two holes into the end of each of these boards by drilling through the frame on either side. Attach the boards together with 3-inch wood screws.
Stand a 3/4-by-48-by-48-inch plywood board up against the front of the frame. Working from inside the counter, drill holes every 12 inches through the plywood board, being careful not to pop out the front side, which is the front board of the counter. Attach the boards together with 2-inch wood screws, again working from the inside.
Set a 3/4-by-18 1/2-by-48-inch plywood board against each side of the frame and follow the same procedure to attach these side boards against the frame -- so their edges are flush with the front board surface and the rear of the frame.
Flip the counter upside down. Position a 3/4-by-18 1/2-by-49-inch board across the bottom of the frame. Install this bottom board using the same procedure as the other plywood boards.
Stand the counter up once more and position a heavy countertop of at least 1-by-19-by-50-inch dimensions atop the front counter frame. Drill up through the inside of the frame and into the countertop, being careful not to damage the countertop surface. Attach them together with 2-inch wood screws.