I have horses and I choose out of convenience to feed round bales in lieu of square. Basically a horse will stand and eat way more than they need if given unlimited access to hay. This is expensive and the only benefit is additional manure for the garden.
I came up with the design in the pictures. The box is 6.5’ x 6.5’ to accommodate the largest bales out there. There are 2 doors that swing outward for loading hay bale and the other 2 sides fold down. The sides are lined with standard cattle panels and the horses reach in and pic out smaller bites of hay at a time. As they eat the bale, gravity pulls the sides that fold down onto the bale. I added a 12’X12’ roof to keep the bale dry and mold free while they eat the bale. There is also an axle and removable tongue for moving the feeder around the field.
I built this a few years ago and I would consider it a huge success. By placing a bale on the ground, the horses will eat through it in 5 days. When using the feeder the bale will last 9-10 days. That has reduced my hay cost by about a $1,000 per year.