If you're willing to drill screw holes into the cage (carefully checking its position depth first), you can get an extremely solid mount by screwing the cage directly to the vehicle's mounting bracket.

This is what I did on my Honda. In the photo, you can sorta see the screws...

The trick is to get screws that have a low clearance on the top so that they don't interfere with inserting/removing the sled. Then screw from the inside out, using the openings in the cage to insert your screwdriver.

Note that I was able to remove the vehicle's mounting bracket from the dash itself (after removing the plastic trim, 4 screws released the mounting cage), but it might be harder for you if you can't do that on your car.

Also, when drilling the holes in the cage, cover up the connector and move the empeg and other parts out of range, so that metal shavings don't get into the connector or the empeg.

I did this on both my Mk1 and my Mk2 installations, and it's proven very solid and reliable.

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Tony Fabris