I did some serious Googling on this one, and came up empty handed. I can surfice to say that I am 99% sure there is no "over the counter solution" to this one. I did find that here are TONS of Cat5 weatherproof cable suppliers!
I think you are most certainly gonna have to use a standard weatherproof electrical gang box, and modify it by mounting a cat 5 outlet inside of it.
Ok, am I the only one who is currious to ask that if you are not using this for a network, what are you using this for? Also, when you have the cat5 pluged in, the box is no longer sealed, and thus not weather proof, only when the box is closed. Is this something that you will just want to plug into now and then, or something that will be plugged in permanently?