I've tried four styles of hay manger and two for graining the babies.  I have a friend with one other style.  I still haven't found the perfect solution, but I'm zeroing in on it!  What you choose will have a lot to do with how your barn is set up and how much space you have, so I'll list what features I like and which ones haven't worked well so you can pick & choose for your location.

1.  Feeding from the aisle.  I love being able to walk along the aisle of the barn and drop the hay into the mangers -- it's efficient and fast.  Easy to clean, which is important because goats will stand on the edge of the manger, get manure in it, and then reject all future hay.  Also, when the goats push the hay out of the top of the mangers (they root around to get the best pieces and that involves pushing the sheaves all over with their heads), it falls into the aisle and (as long as the aisle started out clean) I can just pick it up and feed it again = huge reduction in wasted hay.

2.  Covered managers.  Reduce wasted hay because goats aren't pushing it up and out.  Much harder to fill and clean.  You can make a hinged top, but be sure (a) it won't fall on your head as you fill it and (b) the goats can't open it because they will injure themselves or another goat when they drop it again.

3.  How deep?  Most of my mangers stick into the aisle, so they can only be about 9" deep.  This is really too shallow -- the goats stick their heads/noses in, pull out a big mouthful and drop half of it as they pull out & chew.  I've recently built two deep mangers for the buck & wether (18" deep by 2' wide) and they are much better.

4.  How high should the sides be?  There's no perfect answer -- 3' would prevent the goats from throwing hay out as they root around, but then how do you fill and clean it?  I go with 'just a bit shorter than my arm' so that I can reach in and clean without doing aerobics.

5.  Head holes?  Keyholes aren't great -- goats are violent to each other and a goat with her head in a keyhole is terribly vulnerable.  That means that the most submissive either won't eat well, or will get badly beaten (especially bad idea if they are pregnant).  I prefer a straight hole about 5-6" wide and 12-18" tall (6" if you have a doe with a really big head, and for bucks).

6.  How high off the ground?  The books all say goats like to stand up on their hind legs to eat.  That's certainly true when they're grazing, but my does show a strong preference for the lowest of my mangers - they can just stand and eat.  However, because I use a deep-bedding system (more about that later), I have to build the mangers so that they are "stand on your hind legs" when the stall is freshly cleaned and "stand flat" by the end of the winter.  In order to make that work, I have to have a 2x4 nailed to the wall below the manger so the does can rest their front feet in comfort.