Hi there, so sorry this didn't work out for you. I took one out to play with and remind myself what it's like as I haven't used one in awhile. So thought I'd share a couple of thoughts.
I'm not quite sure what you meant about the opening... yes, it doesn't fasten, but so far as I can tell the opening doesn't affect the bead pack once it's all the way in?
Regarding hygiene, just as a suggestion, some people use a tissue or one of the hygiene sheets that some of the mask makers are selling now, so that the outer cover doesn't have to be washed so frequently.
Regarding sliding around, I understand what you mean, and I think it' the beads themselves. It's fairly loosely filled. I am thinking that perhaps a gel-filled mask type - or, alternatively, a USB mask might do a better job for you in that regard? All of the bead-filled ones I've come across are pretty similar to each other. Let us know if we can help you find something that better meets your needs, and again, sorry this one didn't work out!