I couldn't find a good place to keep my garage door opener. The sun visors in my car were thicker than the clamp on the opener, so I took the clamp off and tried various spots to keep the opener. I came up with the idea of removing the electronics for the opener connecting a switch and putting it somewhere. There was a handy storage cubby between the seats that was really too small for any other use anyway. I printed this cover on my 3d printer and added in the storage slots for USB sticks which happen to be how I listen to music in my car. Here's how to make the same thing (you could modify this to fit any sort of cubby in any sort of car). The original designs I've posted here happen to fit a 2015 Mini Cooper, and possibly other Coopers.
This was really a very easy project and the only difficult thing would be designing your 3d part if you need a different one. Ok, there is also a teensy bit of soldering, but it's easy soldering as soldering goes.
Step 1: Get Prepared
You'll want to find a likely place to mount your completed cover plate. It needs to be somewhere that the innards of your opener will fit.
You'll need a few parts:
1 x switch
2 x wire piece about 3-5 inches long and around 18 guage.
1 x shrink tubing that will fit your wire.
1 x garage door opener
1 x cover plate of your choice (3d printed like in this guide, or made any other way).
Here's a link to my cover plate design: http://www.thingiverse.com/thing:1501499
Step 2: Get Going
Remove the opener's electronic components from the case they started out in. Some openers are easily disassembled by removing the screws that hold them together. Some, like mine, are held together by clamps that are integrated into the shell. However, they should all be designed for a method of opening them to replace the battery. Take your time, if you damage the case, no big deal, but you don't want to damage the electronics.
Figure out your switch mounting. In my case, the switch is just stuck through a hole and secured with a washer and nut. If you have a different type of switch, you may need to choose some other type of mounting.
Step 3: Wire It Up
This might be a little tricky, but figure out which button on your opener is the one to open the door (maybe you only have one button and that's easy). I had three buttons, but only one is used for opening the door.
Now the tricky part. You should be able to see the button leads poking through the circuit board and soldered in place to hold the button to the board. This is where I can't really tell you exactly what to look for, but you are looking to find where those solder points on the button connect to traces on the circuit. My button had four leads that were soldered, but 2 were just to hold it in place and 2 were connected to traces. Once I had identified the connections that mattered (electrically significant) I soldered a wire to each of these points. You can see these soldered connections in the 2nd image above. It's a bit messy, but it works perfectly fine.
At this point, you should be able to touch the other ends of the 2 wires together and open your door (a little insulation would need to be stripped if it hasn't already been). This is manual switching, but once you connect the switch to the wires, you have switch switching.
The switch I used is a momentary contact switch, which means it's only making a connection while it is pushed. This is the sort of switch you most likely need but perhaps other types could be made to work.
Before you connect the wires to your switch, slip a piece of heat shrink tubing over one of them. You could cover both if you like, but one is sufficient to prevent them from bumping into each other and switching when not expected.
Solder up the switch ends of the wire to the switch, then shrink the tubing over one or both of the connections (see photo).
Step 4: Finish It Up
Assemble your switch into the cover plate. Place the whole assembly in your car and enjoy!
Note: you might find that getting a cover you're happy with takes a bit of trial and error. That's how I made mine, just have fun with the process. Also, you may decide later that you need a fancier switch or some other modification. That's ok too.
Finally, I didn't modify the car at all, so none of this is permanent, but you are free to do what you see fit.