The donkeys have had the slow feeder for a couple of weeks now. They have learned to pick up the lid and reposition it! Ringo can get his teeth around the metal grid, pick up the lid and put it down again on an angle so that it no longer completely covers the hay ... amazing! He can't remove it, but repositioning it defeats the purpose of the slow feeder.
They have also figured out a way to coax rather large bites out at one time.
The next model will have a smaller grid ... 2 " x 2" instead of 2" x 4" and the lid needs to fit inside more tightly. This would work better on a container that is the same width top and bottom. That seems hard to find, so a custom built box may be the answer.
It also needs to be bolted in two places to the fence or stall or it needs to be so heavy that the donkeys can't budge it.