Ok, how much room you got? The one I made is the most simplest yet (i feel) the most natural way of making a shifter. Picture this: -
It comes from the floor to whatever height you want. It involves a few pieces of wood, a longish metal shaft, spring, gamepad and 6/7 switches.
In basic terms you have a square piece of wood at the bottom with a hole in the middle, place the metal shaft in here. Then get 4 pieces of wood and cut them to the size of however high you want the shifter, these are your uprights.
Then make an exact copy of the wood on the floor but this time cut out the shifter pattern. Put a small nut on the shaft then place a spring ontop to stop it falling, then put a smallish metal plate ontop of the spring, making sure its bigger than your width in the shifter pattern. Then place the piece of wood ontop and screw it down. Make sure everything moves smoothly and the spring keeps the shaft in place.
Now take off the top piece and dismantle a gamepad. Wire everything up as stated in an earlier post and connect the other end onto a push switch (must have a leaver). Now glue/nail these onto the bottom on the wood so that the lever just pops out enough past the shifter space so that when the shaft is in the 'gear' the leaver is being pushed in. Place the wood ontop and get all the wires out of the way to the side. Connect the gamepad up and there you go!
Simple and easy to do takes about 2 hours (if you know how to solder) I just made it quickly and now planning to make a much neater version soonish.
If you want pictures just say but i'm in work at the moment.