Thermos.
Basically, put your hot supper into your (hot) Thermos in the morning, and if you're at work at 6, eat it and if not, take it home and your supper is already waiting for you.
Alternatively, bring a double portion of cold food for your lunch, heat it in the communal kitchen, dump half of it into the pre-heated Thermos, and eat that wherever you are at 6/7.
I know this doesn't work for the "leave in desk for a few days at a time" part but it's one more route, and allows you excellent nutrition on the cheap, nice warm cozy meals on long days and, on shorter days, a pre-made hot supper. Soup, stew, pasta, etc. Lives nicely in desk drawer for one long day.