Call your local Tractor Supply, Atwoods, Rural King or whatever you have there. Ask about the cow/hog panels, but ask them about the horse type panels, they have 2"x4" holes, are 4 gauge wire, are very strong. They can be cut to size and welded. A large dog would have a very difficult time chewing it.
Your welder friend can guide you on how to frame it out. Angle iron 1 1/4" x 1 1/4" x 3/8" would be a good size. That means the thickness of the metal is 3/8" thick and it is 1 1/4" wide on both sides. Or he might weld it to flat bar stock, thus eliminating the flange on the angle aron
The 2"x4" horse wire on the roll is strong also, we fenced our place with it. But any dog that could chew through the 4 gauge horse panels would be one big, scary beast. I don't think even a scared, panic stricken, large dog could get through it.