you haven’t mentioned if you’re flying internationally, or even what country you’re in now, lol. That info would help.
The problem with checking them in your bag is that you don’t get an opportunity to explain what it is, if there’s an issue.
Is there a reason you can’t encapsulate the powders yourself? I’ve capped powders before, it’s not that difficult. if you do that, you can put them in a supplement container and likely not have an issue.
as far as plastic bags go, honestly that’s what I usually use. I just throw the supplements (capsules) I need into a bag for the week or whatever, and that’s sufficient. I take them in my carry on, I never check bags.
Side Note about flying: I actually never check bags, period. I always mail the stuff I need ahead of time to my destination (hotel or wherever I’ll be). Statistically speaking, checked bags with airlines are significantly more likely to be lost than packages sent with either UPS or FedEx. And with Ups and fedex, the contents of your shipment undergo far less scrutiny than checked bags on an airline. Something to consider. It’s also usually less expensive.