Try canned crushed tomatoes. If you can access some seasonings, a bit of garlic powder and dried Italian blend herbs go a long way to making it taste more like pasta sauce. It can be warmed in the microwave or stovetop if you have access. It is safe to eat right out of the can without heat but def tastes better if you can simmer it for a while. Leftovers will require refrigeration or can be frozen. You mentioned that you can’t keep produce, so if you don’t have a fridge or freezer, look for small cans of plain tomato sauce - not pasta sauce, and not tomato paste, but just plain sauce. You can dress this up with herbs also and have a smaller amount to avoid leftovers