I have a selection of low fat, low cal snacks in my larder all the time. Have a wander around your local supermarket and you will be surprised what you can find.
For example, last night for my late night tummy rumbles I had a Asda's "Mighty instant chicken noodle" instant soup, cals = 107 fat 0.3, it was warm, tasty and did the job. The other thing that I have found a life saver if a new range of "Mug Shot" noodle snacks. Made in a mug there is a range of flavours and the cals are about 210 fat 3.5. Again so easy to make and really filling.
My other standby is a pack of Aldi's "Benefit" cereal, especially the one with red fruits in and topped with home made yummy low fat yogurt.
Hope this helps, good luck.