So I am having a welder make a 24'x48' frame, on the bottom of the frame I would like to have 3'x3' metal plate about .25cm to 1cm thick on each corner to act as areas for legs. If I had him leave it as a plane 3'x3' square how could I go about making attachable/removable legs. If I drill a hole in the metal plate I can only go the thick ness of the plate plus an additional 1/4th inch toward the frame. The width of the frame is 1 inch as well just to complete the specs of the frame. The legs can be wood/ any other material.
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The Corner of the frame and 3'x3' plate

Maybe have the Welder provide a 3 1/2" Pocket at each of the corners, This could allow 4X4 Pressure Treated lumber to fit into. Could easily change height of Legs at any time. Could have metal legs fabricated instead. Gravity will Hold Legs in place. Even add a couple of screws to hold in place if needed. I would give the Welder a 4X4 to make a tight fit. Good Luck.

Typically, table legs that don't have an apron (to hide blocks and/or metal brackets) use T-nuts in the plate and a straight bolt in the end of the leg.