Depending on where you plan on putting your feeder, how many alpacas you have and what you plan on feeding will go into what your feeder must have.
If you plan on feeding your alpacas outside exposed to the weather, then having a top "roof" on the feeder should be added to keep food dry.
For feeding hay, there should be a cover that lays over the hay and pushes it down with holes are spaces in it for the alpacas to put their mouths and chew the hay. When you just throw hay into a feeder and have it poking out all over.. the alpaca goes to take some bites and they dont close their eyes when going to eat so the hay could poke and infect the eye.. so not only does it keep their eyes safe, but it will keep them healthier.