Can any one help me with the installation of new gaskets for the tail lights on my 68?

I cannot get them to not leak. It seems that tightening the housing just doesn't smash the gasket enough to make a complete seal in the corners.

The body is nice and straight and I just cant visually see what the problem could be.

I test by running water down from the top of the tail light as it would if it were to roll down the trunk. You can definately see it dripping from the corners.

Anyone else have this problem?