There are three systems available in the BMW catalogue for the Z3 hardtop, and the tops must be very close in size since both cars are almost identical in height & width. A rack on wheels, a pully system for raising and lowering from a ceiling mount and a "bag" with handles that can be hung on the wall with supplied hooks.

My personal instinct is to hang it up out of the way so nothing bumps it.

Bob

BTW Minimarvel, the octaine of premium here in CA is "92" which is all I use in any of my BMWs.

Is the rack for your roof a BMW product or aftermarket?

BMW supplied (and appears manufactured product)
comes as standard with the car....when you get it!