Have been rehabbing my sis's barn this summer, just made, oh, 7 barn doors & repaired 2.
Two were 8x9 double splitting, one was 4x7 swinging, and one was 4x7 swinging we made top & bottom. These were all wood, outside had to match the 2 exsisting doors. Layer of drop siding turned inside out, a framwork of 1x10 & 1x6 in the middle, & drop siding on the inside. These doors can handle direct contact with cattle, horses, etc.
Upstairs we put a 4x7 sliding door & a double slider on a 10x10 openning. These are car siding vertical, with 2x6 framework.
For your size openning, the sliding track is the best idea as you have. You can make one large door, or split it in half vertically, one slides to each side of the openning. I would favor the split doors, but depends what you have room for.
You can make a frame of 2x6 and cover with wood or with metal. I personally prefer the wood if it is around critters.
I know I don't have much details, but hard for me to put construction bits into words - it just happens, I can't detail it.
--->Paul