Make corner posts for each box: Using a circular saw, cut eight lengths of 2x4 to the height you want the finished counter to be, minus the thickness of the countertops and the height of the metal post standoffs. Using a drill/driver, screw the 2x4s together in pairs with 2½-inch deck screws. Using a circular saw, cut a 1½-inch-deep-by-3½-inch-high notch at the top and bottom of each post.
Separate the posts with stretchers: Cut four 2x4s to the depth of the box. Line up the four posts and screw the 2x4s to the top and bottom of the posts to tie the sides together. Cut four 2x4s to the width of the box, minus 3 inches. Run these between the posts at the top and bottom of both the front and the back of the box.
Wherever you will have cabinets, put a 2x4 for support in the middle of the bottom framing.