I also use the Cashel Feed Rite bag that Tex linked. For powdered supplements, I add a splash of water unless the feed itself is sticky (like Triple Crown Senior), so it will bind. No problem!
That said, I’m not sure a feed bag will solve all your problems. First of all, even though a more dominant horse can’t chase your horse away from his food, they can still chase him or try to grab the feed bag off his face! I have a dominant horse who just gets a ration balancer, and I sometimes have to intervene when he finishes his boring grain then menaces the hard-keeper who gets lots of yummy senior feed. I’m pretty sure if I didn’t keep him in line there would be an altercation. So if your horse is low on the totem pole you may still have issues, even with the feed bag. You also don’t want him choking because he’s bolting his feed when the others hassle him. The pen really would work better.
Second, like others have said, a feed bag will only help you if the staff will use it. Since the pen is already there and they’re not using that, I don’t know if they would use a feed bag either. The real problem is that they don’t think there is a problem, so they’re not interested in solving it… Approach the BM like Trubandloki suggested (not accusatory!) and be willing to pay for him to be fed in private, one way or another.