It's not a matter of making it fit tighter, the seal is not meant to keep
water out. More likely one or more of the drain tubes are clogged so water
is backing up in the internal gutters.

Copied from The Brickboard FAQ:
http://www.brickboard.com/FAQ/700-900/BodySunroof.htm#SunroofLeaks

Drain Holes:

[Response:  Tony Stanley] If its a factory fitted sunroof the 'seal' is not
to meant to be a perfect seal and the gutter should carry away the water.
The chances are the drain is blocked, or the hose is kinked.  They drain to
the front and rear.  The front goes down the A pillars flowing out behind
the front wheel, the rear goes out the C pillars and down the rear quarter
panel, flowing out behind the wheel.  [Don Hodgdon] The drain holes for the
front sunroof drains are just below the leading edge of the front door. If
you feel under the rocker panel, there is a "D" shaped hole that the water
from the sunroof and the fresh air vent (at the base of the windshield)
drain to. they can be clogged with undercoating. Water from the sunroof's
rear drain holes goes back through plastic tubes, down the C pillars,
through the trunk (you have to peel back the carpet on the sides of the
trunk to see the hoses) and out through a plastic fitting mounted to the
lower quarter panels, hidden on the outside by the rear bumper trim. Debris
from the rear window drains clog up the plastic fitting which can flood your
trunk and cause the sunroof drains to back up. You can disconnect the hoses
and use a shop-vac to clear out any stubborn debris. [Editor] To clear
blockages in the tubes, use a long cable with a non-snagging end so you can
rotate it within the tube as it is pushed down from the sunroof. Compressed
air might either compress the blockage or burst the tubes apart deep with
the pillars.