The house rabbit society would have you believe that a rabbit must eat it's weight in hay every day to stay healthy.
Many rabbit breeders simply don't feed hay, finding it a messy waste, ergo a waste of money, and unnecessary as their rabbits stay healthy.
Both maintain healthy rabbits. Don't you find that incredibly intriguing? Completely opposite thought patterns, yet rabbits that are hale and hearty.
My approach is to use hay as the gut stabilizer that it is. Giving hay during times of transition and potential stress. Otherwise I find alternative sources of fibre (which is why people use hay, for it's long fibre content).
Black oil sunflower seed is one source of long fibre, as are weeds, grasses and various plants.
For people who are allergic to hay but not to grasses and various plants as well as black oil sunflower seeds, it means they can also keep a rabbit. They could also feed straw, corn stalks, and hay cubes.
Use a feeder or not?
Homemade or purchased, hay feeders require rabbits to pull their hay out of the feeder. This of course means you need to stuff the hay in. I've never found a hay feeder that that didn't result in some waste simply in the process of filling it. :)
The advantage to a hay feeder though is you can put what the rabbit needs in it so they can nibble over the course of the day. Assuming that unlike MY rabbits yours will nibble over the course of the day. Anytime I give my bunnies hay they gobble it all up as fast as they possibly can. They then sit on (if they get full) and poop on any remainder. Rabbits are rather silly aren't they? :)
The advantage to just tossing it in the pen is that you don't lose hay trying to stuff it into the feeder, but rabbits are more apt to sit on it immediately. Doing so can cause them to foul it fairly quickly.
What type of feeder?
I do not recommend those wire ball feeders that are often sold inexpensively. Some rabbits are silly enough to get their head stuck in them and if that happens while you are away, your rabbit may perish due to strangulation or shock.
To be honest though, I'm not sure, outside of the 2 in 1 feeders that I would recommend most of what they have on amazon. And those ones I'd want to put up on a brick so the rabbit is eating higher up. I find they waste less feed that way. Even to hang it up beside a rectangular litter box.
The best hay feeders I've ever seen are the simplest ones. Cutlery drawers, kleenex boxes hung outside a wire cage, circular wire ones built into a corner of the cage, etc. Something that is less fancy and more functional.