I agree with Hazelrunpack--
to get a visual image of this, you can go to here:
and look at the "dog proofer fence extender system"
I don't know if you can do that sort of thing as a DIY job, but if you have enough photos you can probably try :-)
...or of course you can just buy a kit from them.
I have my eye on the cat fence and might buy one later this spring. If you do end up buying something from them please post again and let us know how it goes.