You might be able to use a truck I use for installing toilets:

Greased tape and silicone.

For the toilet I put down some extremely thin mylar tape on the ground and use some grease like cooking spray or super lube on that.

Then I position the toilet with some small shims like cooking skewers to keep the toilet 1-2mm off the ground.

Squirt in the silicone around and let out set up a couple days.

Lift off the toilet, peel off the tape and the bottom of the toilet has a perfect "fingerprint" of the tile floor including any grout seam etc.

I can put a tiny dab to fill the shim spaces or leave for weep holes.

You could do similar to make a perfect fit that would lock in like a Lego but you can still pull out. Put the greased tape on the car side and the silicone bead would be on the head unit.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)