I asked Hugo about this a long time back, as I discovered a "free" hole in the back of the unit case. I cut and mounted a small eyelet at 90 degrees to the case with a thread locked M4 allen screw. For this to go through the sled when inserted I cut a vertical slot in the rear panel of the sled and checked that it went through OK. I then went out and picked up a washing machine door solenoid with a length of about 2 cm and a bolt thow of about 0.75 cm (it's a Miele washing machine part) and mounted it on the back of the sled. It is wired through a high current resistor to the ignition AUX so that the solenoid only withdraws with the ignition on. It does have a thermal bimetallic strip inside it so that you actually have to wait for a few seconds after turning the ignition on before the bolt withdraws and you can remove the unit. This means that when the ignition is off, the solenoid engages and locks the unit in the sled. The solenoid plunger/bolt was shaped a bit with a steel file to produce a "wedge" profile, so that when the unit is inserted, the plunger is depressed out of the way before re-engaging through the eyelet. This worked quite well.

The problem with this setup is that the eyelet and screw attached to the unit are necessarily small, and not very strong. A single M4 screw, even if it was tensile steel, is not up to much. The case and the insert are also suspect, as there is no easy way to judge the strength of the setup. I was a bit loathe to test it (as I am sure you can understand ), but I am certain there will be boyos with no such qualms when it comes to a real "nick". For this to work properly, the case and sled would need to be designed for this setup from the start, and I am not about to void my warranty to try modifying the case.

In the end I just took it off and learned to take the thing out of the car each time; having said that, this weekend I forgot and left it in the car for two days (!!!!!), then remembered late on Sunday night. I went out to retrieve it and was promptly stopped by the local police for attempting to steal it!