If you have a horse that is prone to choking, this simple slow feeder works much better than adding rocks to your pan.
It is made with pvc caps for sewer clean outs, zinc bolts, washers, and nuts, and a rubber feed pan. The horse this one was made for was a pig who bolted down his feed but I also have made some with one cap right in the middle. Drill holes as needed through the rubber pan and pvc cap, insert the bolt with the head of it in the bottom of the cap, turn the pan over and put a washer then a nut and tighten down snugly. You can also do this with a plastic feeder that is mounted to the wall of your stall.
TIP…make sure your bolt isn’t excessively long. You may have to tighten the nuts occasionally depending on what an avid eater you have!