I retrofitted a MMI 3G+ to my A6 and used the slot in the glove box to install the unit instead of the boot, this is a panel to cover the unit so it looks closer to OEM look. The slot is a tight fit and helps keep the MMI unit still as well.

I installed the deeper slot to the top of the unit, but it works fine either way, if you could find a felt which matches the glove box and stick it to the cover then it would improve it further.