I stronglly recommend putting them through the wall or in a window as part
of a program for keeping vermin from using the flap.   This is especially
important in relation to opossums and raccoons.

My solution can be seen at
<http://67.54.246.139:9000/tdavis/cats/cat_flap.jpg>

The opening on the right was the original flap, but it was an invitation
to coons and possums, the one on the left (with Tigger on the shelf) is
the current version. The shelf is about a meter from the top step - there
is a full lenght shelf on the inside, one of three above and below the
window. The idea is that the wind will always press one or the other of the
flaps harder against its rim, regardless of whether the air flow is inward
or outward.

The flap is my own design and is still under development, but the basic
idea is *two* flaps, each of which seats (with overhang) against a raised
rim framing the hole - the cats open it the same way they open cabinets:
by pulling it open with a paw.  The hinge had to be modified by replacing
the hinge pin with a smaller piece of brass rod to make the hinge very
loose. The flap itself, in its current incarnation, is a piece of Lexan
with a thin strip of wood glued at the top and a a heavier on at the
bottom.  <http://67.54.246.139:9000/tdavis/cats/cat_flap2.jpg> shows this
and the detail of the removable hinge pin (cleaning is easy: remove the
pin, take the flap inside, and wash it, then put it back and reinsert the
pin).  You can also see the coarse lining of the hole with the same
paterial the wall studs are made of.  This is an unfinished design, still
very much under development and testing - the hole lining is unfinished
and the indoor flap is not installed.

After several years of testing various modifications, I can say that the
filal version will use either brass or stainless steel for all metal
parts, a smaller wood block will be used at the bottom, and there will be
some sort of shield to keep freezing rain off the hinge (even though the
flap and most of the shelf are under the eves, blowing rain still gets
to it, though none comes inside except on or with cats). I'll use maple
for the wooden parts and finish them with spar varnish or heavy duty
paint. It is important to provide the wood block at the bottom so the cats
have something they can easily get their claws into.

BTW, in the second picture (taken a few minutes ago), you can perhaps see
the contact area between the flap and Ozy's nose - he's about to push it
open from the inside.

The concept can easily be adapted for use through a window in place of the
screen and lower sash - as long as the cats don't have to leap much more
than a meter up or down.

Of my fifteen cats, only one won't/can't use the flap, but that's
understandable when you consider that he is at least twenty years old, is
very frail, and is unstable on his feet.