I'll have to see if I can dig one up from when I set it up, or take one as soon as I break it down to move it. My over flow is approx. 8"L x 4"D x ? (forget the height of the tank, LOL. It's about 1/2" below the lip of the tank.
What type of rubbermaid stock tank are you referring to? Stock tanks are usually really large (livestock), the brute rubbermaids are used for sumps, sometimes. I have seen them after a short period of time crack and split due to the constant water pressure. If you or someone you know knows how to work with acrylic you can make one. I had someone make my overflow and sump. Guy was out of Atlanta, he did custom work, not anymore though. Tom, I believe his name was. He did custom work for Star Aquatics & Blue Planet, and others I'm sure.