I'm sure they'll vary a bit from car to car, but mine measures about 3/8" gap at the center rear, and fits tightly with no leaks.
'97 SL500, 40th anniversary edition
'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)
SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)