Ideal Cat Flaps have polycarbonate flaps with a magnet to hold them shut and a 4 way lock that allows for open, locked, in only or out only.
These inexpensive cat flaps are durable, but not very weather tight.
There is an electronic version of the larger size which can be purchased on this page, or you can go the E-Cat specific page for more details. The flap is big enough to fit a medium size dog up to 35 lbs or so. This electronic version is not raccoon proof, but it will keep out strays. It will not prevent your indoor cats from getting out while letting the dog out.