Anyone who reads any of my posts on this topic already knows what I'm about to say--porta-grazer w/ grass hay, no grain or other concentrated feeds. We use AZ Copper Complete, an arbitrary decision based on where our Bermuda grass hay comes from, and offer that supplement free feed mixed w/ equal part loose salt--I put out about a week's worth per horse at one time, and replenish it weekly or sooner if needed--since she's in a stall by herself, you can just put it in a bucket or similar feeder attached to the wall (we actually keep flat rubber feed pans in each horse's run-in shed, just in case it ever rains here again). I'm sure that there are vitamin mineral supplements that may also be good, but ideally these should be based on the nutrient content of the hay, which means testing a LOT of bales using a core sampler--probably not practical in your situation. My friend who buys her grass hay where we get ours tried to convince them to test their hay--absolutely NOT--they wouldn't even discuss it. Grrrr So we just give it our best guess.
I come from a background of alfalfa hay twice a day, accompanied by a 1 lb. coffee can of sweet feed once a day if the horse is in work. I've realized in recent years that a person can learn a lot by keeping an open mind. People in our area probably think we're stupid, but we swear by our Porta-grazers, which we've been using for about 9 months.