I love your idea for decorating the front of your feed store, although I have a question for you: why just use the trough for a planter? It seems to me that horse people might like the idea of hitching their critter in front so they could try bits, bridles, saddles, or whatever else you may sell on for size. Also, folks trailering past your place might like to give their stock a break and a drink of water while throwing some bags of feed in the truck. (If the horses should happen to leave a few "road apples," just compost them and you can add a line of fertilizer to your inventory!)
If you want to be really "old timey," you'll need to find a hollow log and cap the ends with a couple of boards. Or, you can do like the logging camps in my neck of the woods used to do, and hollow out a log with a hand adz or broadaxe.
As places got more established, the watering troughs were often constructed of cast concrete. This had several advantages: Horses didn't chew on it, it didn't leak as much, and the water that soaked into the concrete helped cool the other water a little through evaporation.
As time went on, galvanized steel tanks became popular, and still later, rubber and various types of plastic.
What I suspect you're looking for, however, is something like the trough in the color photo taken at the Wallace Cerri Ranch, Paradise Valley, Nevada.
Here's how I'd go about trying to duplicate that trough:
Take three 2"x12" planks (which are really about 1-3/4" by 11-3/4" depending on how they are planed) -- the length will depend on the size of the hitching rail you plan to install it under -- and cut 10 inches off the end of each. (If the planks are a bit weathered, so much the better.) Nail the three planks together with the upright planks on the outside of the bottom plank. Nail one of the 10 inch pieces a couple of inches from each end of the trough. Next you'll need a couple of 16 inch long bolts. Drill a hole big enough for the bolt in the center of each of four pieces of heavy strap iron. Also drill the same size holes in the middle of the upright planks about 1-1/4" from each end. Run the bolts through the holes in the strap iron and the uprights, add washers for spacers if needed, and tighten the nuts. Now, take your chain saw and chop some 15" lengths from a piece of landscape timber. Use these hunks of wood as supports to hold your trough up off the ground.
If you want it to really look authentic, you could take an axe and rough up the top edges to look like horses had been chewing on it.
As a finishing touch, mount a pitcher pump on a pipe at one end of the trough. Instead of using the trough as a planter, you might also consider caulking the seams to make it more waterproof, filling it up with water, and adding a recirculating pump to cycle the water through the pitcher pump. The sight and sound of water flowing out of the pitcher pump into the trough might be real refreshing on a hot day.
If any of this is a bit confusing, a close look at the right side of the color photo should help clear things up.
I hope this helps -- I'd really like to see a photo of your finished project!
(Photos courtesy of the Library of Congress.)
