A) Cut the plywood into two 8'strips, one 30" wide, the other 18" wide. Cut two 30" rectangles out of the 18" wide strip.

B&C) Clamp 8' 1x2s, on edge, along the long edge of the plywood, and screw them into place through the top of the plywood, using 1 3/4" sheetrock screws. Cut shorter peices (approx 28 1/2") of 1x2 to fit between the longer rails, and screw them into place also. This should create an 8'x30"x2" shallow tray, with the edges of the plywood exposed.

D&E) Clamp 8' peices of 1x3 alongside the 8' 1x2s' on the outside, so that the top of the 1x3 is flush with the top of the plywood, concealing the cut edge. Screw into place using 1 1/2" wood screws, from the inside. Cut 1x3s to fit along the short ends, extending to be flush with the outside corners of the long 1x3s, and screw those into place, also from the inside. (These should be about 31 1/2". You should now have a heavy-walled tray, 49 1/2" by 31 1/2", about 2 1/2" high, and about 2" deep on the inside.
Put this tabletop aside for now.

F&G) Cut two 8' 1x6s into 2' lengths. ROund off the top corners of each 2' board with a 3"-radius quarter-circle. cut a 1/4" deep 'dado' 18" wide on one face, top-to-bottom, centered in the board. This should be wide enough to accomadate the 30"x18" plywood, and half as deep as the plywood is thick. These are foot-boards.

H) Clamp the top and bottom edges of the plywood between facing footboards, with half-and inch of foot-board extending past the plywood at both ends. Glue and screw these into place, with 5 wood-screws from the inside only, top & bottom. The resulting legs should be 24" wide, 31"high, and 1 1/2" thick.

I) After the glue has dried, cut slots in the legs. At the top, make two 1" slots, 3" long, 1.5" down from the top edge, and 4" in from each edge. If you could see inside the board-sandwich, this would leave 1" of plywood outside the slots.
Cut a similar slot in the bottom, only center it in the foot.

J) Cut three 1x6 boards to 92" long. Make a 3" wide, 5" long tounge, by cutting 1.5" from the top edge, and 1" from the bottom edge, at each end. Starting 2" in from the end, and centered in the tounge, cut a 2" slot, 3/4" wide, toward the base of the tounge.

K) Insert these rails, two at the top, and one at the bottom, into the legs made in H->I, after the glue has dried. The 1.5" belly of the rail should be UP on all three rails, so that the top rails are flush with the top of the legs, and the bottom rails are 1/2" clear of the floor. Wedge the rails in place through the slots in the tounges.

L) Drop the table-top in place on top of the completed trestles. It's weight should be sufficient to hold it in place, but if not, glue wood-blocks to the table-top around the legs, to keep it from shifting.

Paint, oil, and/or carve the result to suit.