This instructable is just one example of how you can make a custom switch panel to add some realism to your flight sim. Is this the best? No, far from it, but hopefully it will show you how easy it is and you can go from there. This panel is very simply based off the DC-3, which is the airplane I fly most of the time. It's really really really really AWESOME to take off and land without using the mouse!
Some things you will need:
Xinterface and Landing Gear Delay Module (optional) from www.FlightSimElectronics.com
Thin plywood, sheet metal, plastic, cardboard (reinforced with fiberglass), or what have you for panel
Switches and pushbuttons
First, draw out on paper how you want your panel to function, with the layout of the switches and buttons. Draw your design on the plywood. Simply use a drill for the pushbutton holes. The square switches you will need to first drill at a corner, then use a jigsaw (or dremel?) to cut the square holes. Paint the panel however you wish, then insert all your switches and buttons. Solder up the switches per the Xinterface instructions (example, an On/Off switch with two posts will have a "column" wire on one terminal, and a "row" wire on the other. On/Off switches require a diode, but that's pretty simple and covered by the Xinterface).
Once you are wired up, you can mount your panel to your desk using some small screws, or you can attach it to a stand of some sort. Just plug in the USB connector, fire up your favorite flight simulator, and assign the joystick buttons (switches and pushbuttons) to the desired functions. That's it! Pretty simple, super cheap, and now you have a new dimension of fun!
You can also do this sort of thing for race car sims, train sims, and even make your own custom joysticks (for FPS's, World of Warcraft, whatever). Use your imagination! Thanks for looking!
UPDATE - I have added 3 potentiometers for Rudder, Elevator, and Aileron trim. I also added LED's for the Landing Gear indicator lights My panel is for a DC-3 so that's why it's pretty rough and basic.