Cut the 4 end rails from the 2X4 16 inches long. 16 inches leaves ½ inch front and rear clearance with 3 ½ inch legs for a 24 inch wide top. Adjust to fit your top. The photo below shows a quick jig I put together. The jig is nothing more than a couple of 2 bys and a square.
Bore the 3/8 holes into the ends of the rails using the holes in the leg as guides.
For the bottom rails, use a guide to insure that the spacing from the bottom is correct. My spacing from the bottom of the top rail to the top of the bottom rail was 29 ½ inches. Use the holes in the legs as guides.
Drill counter bores in the ends of the rails. The counter bore should be centered over the ends of the holes drilled into the end of the rails. In my case the bores were 1 ¼ inch deep and the bottom edge was 1 ¼ from the end of the rail. The bolts are 5 ½ inches long.
Use a chisel to cut the bottom nearest the ends to a D flat.
Assemble by placing a washer on a bolt, slide the bolt through the leg into the rail. Place another washer and a nut on the bolt and tighten.
The end result is two ends for the bench base.
