Measure the kettle grill's base’s dimensions with a measuring tape. The cart’s top should be at least 4 inches larger than the grill’s base, keeping the grill from tipping over during grilling or transport.
Cut two sections of 2-by-4-inch pressure-treated lumber the width of the grill cart with a circular saw. Cut an additional two sections the cart’s length. Miter the ends at 45-degree angles with a power miter saw.
Assemble the length and width sections into a rectangle. Drive 4-inch wood screws through the corners with a drill outfitted with a screwdriver bit.
Measure the rectangular frame’s interior width. Cut one section of 2-by-4-inch pressure-treated lumber to this measurement for each 8 inches of cart length. Place these sections inside the rectangle at 8-inch intervals, and then drive 4-inch wood screws through the rectangular frame into the ends of these supports.
Cut a section of three-quarters inch thick plywood with a table saw to fit the rectangular frame’s outer dimensions. Place the plywood section on the frame and align the edges. Drive 4-inch wood screws through the plywood into the frame at 4-inch intervals.
Cut two pieces of 1-by-2-inch pressure-treated lumber to the cart’s width and an additional two sections to the cart’s length. Miter the ends at 45-degree angles and assemble the sections on top of the plywood flush with the outer edges. Attach these sections with 4-inch wood screws at 4-inch intervals. This creates a lip that helps keep the grill from rolling off the cart.
Apply polyurethane sealant to the plywood, lip and frame sections with a 2-inch paintbrush. Let the sealant dry for six hours, and then apply a second coat.
Turn the cart top over, so that the frame is on top. Position a 4-inch swivel caster on each corner of the frame. Drive 4-inch wood screws through the holes in the caster mounting bracket into the frame.