I use an automatic feeder when I go away, but if you are getting someone else to feed them, individual portions in labled plastic bags or containters, as Ira said, are the way to go (prevents possible overfeeding).
My unsubstantiated opinion of those feeder block things is that they are a waste of time. I think they are mainly made up of things other than food. They are not really appropriate for any top feeders you might have. If you have snails they will eat most of it. They can't be good for water chemistry.
Feel free to crucify me if my opinions are wrong.