This is Walt Ottenad's design for an outboard shifter. He used a piece of oak for the arm and has it attached to the base of his wheel. It extends out from the right side at the approximate location of the real thing. Drill a vertical 1/4" hole through the arm where you want the shift located. Rock the drill bit back and forth a few times to make the hole hourglass shaped. This shape will allow for some movement of the shift arm. Drill a 3/8" hole through the side of the arm that intersects the 1/4" hole directly. This is the pivot point for the shifter and it's where you'll put a metal insert. The metal insert can be purchased at most home centers . It's a short 3/8" metal rod with a slot on one end and has a 1/4" threaded hole drilled perpendicularly through its center. It's really the axle for the shift lever. Thread the bottom third, or so, of a 1/4" metal rod with a 1/4-20 die and insert it into the 1/4" hole. Screw it down into the metal insert until it extends out the bottom of the arm. Slip a section of rubber tubing over the end to act as padding for the switches. You can also thread the top of the shifter to fit a knob of some sort. Each time you pull back on the shifter you'll shift up one gear and likewise each push on the shifter shifts down one gear. This is called a sequential shifter and is exactly like those used in Indy cars. The switches are momentary-type buttons that are on when pressed and off when released. You'll need to fashion some sort of brackets to position the buttons at the proper locations. A diagram of how to wire these switches can be found in the Electrical section of my website.

If you don't want to mess around with dies and threading metal rods, you can substitute 1/4" threaded rod with a piece of metal or plastic tubing slipped over the threaded part between the knob and the insert. Be inventive!
 

In Formula One they use sequential gearboxes the same as in Indycars, except that the shifting is done on the steering wheel via shifting paddles. The levers are located just behind the wheel and can be operated with the fingers, thus allowing the driver to keep both hands on the wheel while shifting. This can be accomplished very easily by simply mounting momentary buttons on the backside of the wheel within close proximity of the fingers. They could also be mounted on the front and operated with the thumbs, but I find that position to be clumsy, especially if the wheel is at a full turn. It's your choice.

But, if you want realism in your wheel, here's a way to construct shifting paddles, just like in F-1!

This is a simple diagram that illustrates the basic layout for shift paddles. The idea is the same as the Indy version, a lever pressing against a button to activate the shift. I used microswitches instead of regular switches in this case as a matter of choice only. A microswitch is a momentary type switch with a small lever that presses against a button. There's a small wheel at the end of the lever, but can be purchased without it. Microswitches yield a distinct "click" when pressed, which lets you know that you've  shifted. I like that. It leaves no room for error and eliminates missed shifts. The paddle can be made from wood, metal, plastic, or whatever else fits your needs. It could be just a stick with a screw in the center for a pivot point, as long as it makes contact with the switches when you rock it from side to side. However, some kind of spring device is needed to return the paddles to the neutral position. My new paddles are made from 18ga. nickel sheet. The pivot is a threaded stud (a bolt with the head cut off) that I epoxied in a hole drilled in the top of the wheel hub. A slot was sawn in the stud to hold a thin piece of spring steel that pressesagainst the free moving paddle. This is topped off with a small washer and acorn nut to hold everything in place. The stud had to be the correct height, so that the acorn nut would not press down too much and bind the movement of the paddles. My first wheel has two separate paddles, one on each side of the hub. In fact, I designed the hub specifically to accommodate them. Try to plan ahead by making simple sketches of what you're planning to do. They don't have to be masterpieces, just "thinking drawings" of you ideas. It's surprising how often you can discover mistakes in your thinking way before they happen. This saves you a lot of time "rigging" later on. If, you know what I mean!

* Here's a slight variation of the paddles above, using two small coil springs instead of one flat wire type.