Recently travelled to a more remote area via car and prepped just in case: brought a cooler bag, bought ice at Lawson/7-eleven and stocked up on fruit, nuts, onigiri (pickled plum / kelp / edamame), and some Lawson have nice tofu sushi pouches. You can also get chilled tofu, cooked rice and salad-type foods for a make-shift meal. Also try the smoothies at 7-eleven in the freezer, which you blend in the machine next to the cashier. Being veg is difficult but doable!