I have a dilema - I want to put a doggy door in to allow the dogs (2 pyrs and 2 dockers) access, but I have 4 cats also, who have always been indoor-only cats. How do I let the dogs out and keep the cats in? Is there a way to stiffen the flap or an inexpensive alternative to the electronic doors? Anybody else dealt with this, what results, any luck at all?????