Ruthy, How about making some nice healthy soup? You will know exactly what you are putting in it and so can count the calories. Where in the summer we have salad and some form of lean protein for lunch on fast days, in winter we switch to soup, sometimes also having a rather heartier one for our evening meal. You will find things like carrot and coriander are very low in calories, like 100-120 for a very large and filling portion. Also you can take a flask of the stuff outside with you. Hope that helps.

How about just a steaming mug of tea? As long as you take the right tea it doesn’t count towards your fast or calorie count (and green tea stimulates fat burning apparently). You can just wrap your hands around the mug and warm up nicely.
In case of the working outside, buy a thermos!