Several posts have suggested using weather-stripping.  Don't see how this
would work, since the flaps are actually flat sheets of foam.

I don't have one in my hand, so measurements are nowhere near accurate, but
they are approximately 2.5 x 4 inches (about the size of a poker card).  The
trick is that in cross section, they have a lip, sorta like this
^------------------^  .  The lip fits around the perimeter of the plastic
flap, or more precisely, flap halves, since two of them snap together around
the spindle, and actually makes the seal.

Thanks,

Karl Wolz