From your description, food doesn't seem to be an issue. Certainly don't drop in a feeder cube; in such a small tank with such little stock to eat it you'd only end up with incredibly fouled water.
As for evaporation, it really comes down to what the conditions are. Here in AZ I have to keep a close eye on my small tanks--between open tops, low humidity, and the attention of thirsty cats I usually top them off weekly. Obviously you won't have the cat problem, and your humidity levels probably aren't as low as here, so it only remains to do your best to reduce evaporation. Cover the tank, make sure to fill it to the top before you leave---and if there's any chance the water level might drop below the proper functioning level of your filter, leave the filter off.
A bit of airline hose, a water jug and a bit of jiggery to get the levels right and it'd be easy enough to set up a reserve container that would add water if/when the tank level dropped too low--but that's the sort of thing you'd want to design, set up and test well in advance of its need to make sure it would work as planned.