I have this problem - possums are even worse.  There *is* a solution,
but it doesn't involve the cat flap all that much.

The solution is to take advantage of the facts that cats can jump, and
possums and coons can't.  Put the hole through the wall at least three
feet from the ground or a nearby platform, then provide a platform for
the cats to jump to in order to get to the flap.  This page about a
raccoon-proof outdoor cat feeder should give you some ideas.

I have a tunnel through my back wall and have been working on
weather-proof flaps.  The general idea is that since cats can open
cabinet doors, they can also open a flap that rests on a support away
from the main surface and has a very loose top hinge.  I found that
the surface that the flap rests on needs to extend farther out at the
bottom.  I use two hinges bolted together to give a really floppy
action.  I replace the hinge pins with smaller brass rod.  One flap on
each side, with weather stripping of some sort (I haven't found the
ideal stuff yet) where the flaps actually rest provides tight sealing
regardless of whether the wind is blowing in or out.  You could just
mount th hinged flap inside the tunnel.  I have found that cats like
their flaps rigid, clear, and clean so I use Lucite and provide a
handle for quickly and easily removing one of the hinge pins so I can
take the flap inside for cleaning, or even replace it with an already
cleaned spare.