Robert, I don't know where you can get the mesh bags (I'm sure someone else here does, however), but I always dry my onions on screens made with "hardware cloth." It looks like your onions are currently in one of the mesh-type flats, which I think would also be fine, they just need good air circulation around them. (If you have another one of those flats and can spread the onions out a bit more I think it would be even better.)
This is a photo of garlic drying, but I do my onions the same way
