according to what i read, a Donkey Kong cabinet uses 20 ft. it is a relatively small cabinet and the t-molding only goes about half way back on the bottom side. i'd probably go at least 30ft.
groovygamegear sells it by the foot, no minimum order..l i ordered 30ft for my other mame cabinet, i havent actually gotten around to installing it yet, but it came in 1 30ft piece.