No need to worry about that, buns will very often just sit very still and quiet in a carrier, it will just leak and make the floor wet. For half an hour I wouldn't give anything, and I wouldn't bother with water for any journey less than 2 hours - I'd just give a leaf of something moist like a spring green for up to that. I've travelled 4+ hours with buns before and they've never had a drink even if I've done a short stop off for them.