My experience is that if one store doesn't have the size weatherstrip you want, try the next one. Seems like every store carries a different selection of sizes, if not brands. Kind of strange, but what the heck.

Thick and wide doesn't equate to watertightness any more than lots of hold-down pressure does. What matters is building accurately, and I suspect this mostly means finishing out the inside of the hatch cover with a glass smooth mating surface that can seal very nicely against the weatherstrip gasket.

If I recall correctly, I used 1/8" x 3/4" wide strip for molding, and chamfered the foam with safety razor blades, which is why I started with such a wide strip. Then I used 3/16" x 1/4" for the actual gasket. I may sound like a broken record saying so once again, but it's almost unbelievable just how little hold-down tension is required to keep a very effective seal.

http://mysite.verizon.net/NGC704/newOI/Page_7.htm