I believe it's not a good idea to keep spare in the back seat over the long term. Depending on the tire compound (?) It will probably stink of rubber for a loooong time.

The 996 compact collapsible spare fits in trunk no problem.

A 18 or 19 FLAT will just barely slip past seats to fit in back inside car.

The Boxster 986 spare will fit the 997S suspension/brakes,it has a mere 3mm clearance from calipers. BUT will have to be deflated fully to fit upright in trunk. Will fit fully inflated if you slant it. (not talking about collapsible 996 spare here).