hmmm, I don't know, these fish are such voracious eaters that I'm afraid you might come home to find one of them in the shape of the pyramid because he swallowed the thing whole.
Seriously though your fish might very well eat themselves sick while you are gone if you use one of those and the truth is they can easily go without food for a week or more so I would just NOT feed them while you are gone. Do a partial water change just before you go and feed them as you normally would and they'll be happy to see you when you return from your weekend .
Another problem with using one of those vacation blocks is that you'd be suddenly changing your fish's diet which is something we try to avoid. Any changes to diet should be done gradually and the new food should be something specifically designed for the species of fish you are keeping.
I agree that your fish will be fine for a weekend. I set up two aquariums in classrooms at the school where I teach and until I recently bought some autofeeders, they always went the weekend without food.
That said, I have to enthusiastically recommend the Eheim autofeeder. It costs about $40 but IMO is well worth it. (I read many reviews before choosing one.) First I bought 2 to take care of the aquariums at school during longer breaks (1 to 2 weeks) so I wouldn't have to keep driving over to feed fish! I liked them so much that I bought one for my home tank. It's wonderful not to have to worry about my fish when I go away on vacation. I even pay a neighbor kid to take care of my home and other pets, but I prefer to set up the fish feeding thing myself. I always worry about someone overfeeding my fish and trashing the tank. You can experiment with how much food the thing dumps in the tank until you get it just right, and you can set it up to feed up to 4 different times a day. I've never been so worry-free since I got these things.