I understand the not having a good vet around, but I would really try to see a vet or at least email one and explain your situation. Unless you already have phenobarbital on hand, you will need a prescription anyway. Also, much to my dismay, phenobarbital is a drug that is seeing its decline in the medical industry and is becoming VERY expensive. If your duck just had one seizure it may be a symptom of an underlying cause and that is what should be treated, not the seizure itself. Phenobarbital is used to treat seizures that don't seem to have an underlying cause and are just part of the individuals biology. Unfortunately, little is known about seizures even in the human medical world. But from my experience, if an animal has a seizure WITHOUT a known underlying cause (i.e. Poisons, nutrition deficiency, organ failures, and many other causes) and continues to have them, it will end up being a spiraling path of treatment and worsening condition over time. Usually the best outcome is management with meds, there isn't a cure. However again, you should seek out what the cause of the seizure was and hopefully it was just a one time thing and can be treated.
All this information is gained from my experience caring for my 5 year old dog who has been having seizures since he was 2. His situation is extreme, but thankfully is somewhat controlled by his medications. I truley hope your duck does not end up with his severity of epilepsy as it takes its toll on both him and I physically and emotionally. But I truley love this dog and we've seen improvement in his condition as well as quality of life. However, I have also spent thousands of dollars on his health.
My advice to you is find the underlying cause. With me knowing very little about ducks still, I have learned that niacin is very important. Try adding niacin (via Brewers yeast) to your ducks food immediately if you haven't done so already. With my dog, I have tried holistic approaches including honey and coconut oil that wouldn't hurt to at least try as well. The science behind these is increasing blood sugar with honey and some protein found in coconut oil that I'm forgetting the name of. My dog loves both of these as treats or on his food. I have no idea if a duck would like these or not though, but it may be worth a try and end up being a fun new treat.
Best of luck to you, and I truley hope this was a one time thing and you find treatment