Bunnings sell sleepers in either 200mm wide X 75mm thick or 200mm wide X 50mm thick, just so you know...might help you understand what size you want for your bench top, as you are confusing me (and I'm guessing others also) with what you want....your original post stated 20mm thick timber being used and now it's sounding like you want 50mm timber, but could be 75mm also depending on sleeper size being talked about.