Ok, real Defender joysticks are hard to find, and are usually worn out, and the rebuild kit for them is $35 + shipping.
But you can modifiy a wico "microleaf" or leaf joystick to be used in a Defender application.

Look at the picture, see, that is a Wico microleaf on a Defender panel WITH the reverse button in the proper location.
Step one.
Cut your panel out. Drill out the reverse button. For the part where the joystick pops up DO NOT drill out a 1 1/8" circle. Instead drill two holes on on top of each other, just BARELY larger than the diameter of your joystick shaft. Then use your drill to connect these two holes together to make the proper cutout for the joystick shaft (which is a vertical slot).
Step two. If your stick is of the "microleaf" type. the pull off the microswitches for left and right and reinstall them backwards (this will stick a part of the otherwised unused microswitches in exactly the right spot to help guide the joystick into a pure 2-way path).
Step three
Install the joystick and make sure it can move up and down.
Notice how your joystick base OVERLAPS the hole for the reverse button. Go ahead and use a 1 1/8" holesaw to cut that part of the joystick base away. DO NOT try to use a spade bit to do this.
You can then install your reverse button without issue. The joystick will still hold in place just fine with the other 3 screws (use screws from the bottom, since there will only be 3 of them). The ring that holds the button in place will also aid in holding the joystick down.