For me a solo 100 km ride requires only two full water bottles unless it is BLAZING hot or it is at race pace. Then I require more water. Greater than that requires some food for me.
If you think that you will need some food try taking a baked potato. It is very bland and packs a lot of calories. It is very unlikely that your body will "reject" (and by that I mean "eject") that. And there is zero packaging to worry about.