If you want cheap and functional, drill a 1/2 inch hole in the trans pan. Place a 3/8 nut over the hole on the outside and weld or braze it to the pan. (For this application, I prefer braze. ) Run the thread with a tap to make sure it is clean. Cut down a bolt so it is about the same length as the thickness of the nut. Use silicone or tefflon tape to seal the "plug" to the nut.
You can put the plug about anywhere as long as nothing protrudes into the pan that could be a problem. Personally, I usually put it toward the rear of the pan on the driver's side.
Steve