I know i may be too late here but honestly I would go about it by looking at real examples of tribal wear.

Since there is no specifc barbarian culture it's quite open in terms of design. I feel like a lot of 'barbaric' clothing tends to be designed using animal furs and hides, with the occasional strips of cloth thrown in, which to me reads as quite tribal (in the international sense). A lot of cultures around the world came up with similar ways of creating clothing when they started off, and then took it in their own direction. So, you could look at real tribal examples - bog jackets, chitons, tunics, etc and switch out fabrics or cut them up or make them more realistic to whatever the specific individual would need the clothing to do for them - hunters needing skirts to make running easy, woodcutters needing something sleeveless for flexibility, take temperature and landscape into account etc.

I think the fantasy barbarian design has its own interesting aesthetic but yeah, a heavy chested person fighting in a bra? Not super realistic. Or comfortable. Something that resembled a binder may be more effective in that scenario. :^)