This keyhole feeder sat on cinder blocks and was screwed to the wall. The inside was too spacious, so we dropped in a board cut to fit - it rested against the bottom front and angled toward the top back. That kept the hay closer to the goats.

Keyhole dimensions:
8" circle with a 4"-wide slot 8" long. The fronts were T-111 plywood sheets cut in half lengthwise and braced every 4 feet. The separate feeder on the end was an afterthought and probably 4' wide. Notches were cut in the backs so they could be mounted with joist hangers (?) to the barn wall. They were probably about 10" to 12" deep at the top and 5" to 6" at the bottom. Very easy to reach in and clean.  

Kids could jump or climb in, but they quit doing that when they grew to a certain size. The child was a visitor.

1" metal pipe was spaced at 6" and holes drilled in the center of a 2x4. Matching holes were drilled in a 2x4 and screwed to the center of the bottom 2x10 - that angled the pipe toward the back on the bottom of the feeder to leave a space for feeding grain in the front.

Feeder ends and bottom were 2x10's, the back was plywood, and the rest was 2X4's.
It was about 24" high.

The open top of the feeder was about 10" deep, and the metal rods angled toward the back to leave about a 4" depth at the bottom.