I usually build my own. Over plenty of years I have come to find that you can do it many ways and for as much as you want to spend, but in the end dependability is the key.
If you have others who come and go at will your might as well plan on it disappearing unless you have a way to suspend it high enough it cannot be reached easily. That in of itself also means you must have a way to reach the top in order to fill it. Remember if you can get to it easily from the ground so can some scum bag.
I hang mine from a limb and use the front end loader on my tractor to raise me up to pour in the corn and minerals. This keeps it high enough the cows cannot get to the motors, and scumbags can't simply walk up and take it or the motors down and haul it off. If it's on legs or low enough to stand on something like a 4 wheeler and reach it, you can bet they will figure out how to get it gone.
I use a piece of 3/4" pipe with a piece of 1/4" x 4" flat bar welded to each end that will just fit into the inside diameter of the barrel. I drill a 5/16" hole in the plate before I weld them on so I can center the hole up on the end of the pipe. Once welded I then tap out the hole with a 3/8"-16 tap so I can then screw in a bolt to hold it in place across the barrel about half way to the top ring on the barrel. This will support the barrel being hung by a chain, and keep it from collapsing with the weight of the corn inside.
Then I measure off enough chain so that I can put another piece of 3/4" pipe as a spreader bar about a foot over the top of the barrel which is about 3" wider than the diameter of the barrel. Once I have this measured and cut I will bolt the chain ends to the barrel.
For motors I use Magnum Hunting Product motors and have found they last and last. If anything does go out it is usually the digital timer. Most of the time I simply replace them with one of the Remington type which will run on either 6 or 12V batteries.
The lids and funnels I use are the spun aluminum type and they will last forever unless you drop the feeder on them. I just drill a hole in the bottom of the barrel and slide them in. Once in place they are good to go. I then use some 3/8" all thread to set the spinner for about 3/8 - 1/2" maybe a touch more from the bottom of the funnel so that the feed will simply sit on top of it without pouring out freely.
Once you put it all together, hang it up on a good 6-8" or larger limb high enough it can't be messed with and filler it up, throw in a battery and go home.
If you are a hustler and scrounger you can gather up enough of this that and the other to build one yourself without breaking the bank. Most of my cost has only been in purchasing the controller/motors. The rest of the stuff I scrounge up here and there. The barrels can be any sized really but I like the bigger ones so I only have to fill them about every month or so. I feed them twice a day but they only through out about 3/4 pound or so. It's enough to keep things coming when they go off, but not empty my bank account keeping them full.
Look at how some of the commercial ones are built and you will find it pretty easy to duplicate their ideas to a point. The key thing is finding a reliable motor/timer that doesn't burn up or clog up with the dust and debris falling from the funnel. That is why I like the Magnum, they are in a closed up box, and there are plenty others out there similar, but I have had 4 of them going every year for close to 25 years with only a change out of the timer.
