I did something like this. I put some extra landscape fabric on top of the plastic bottles (in my case, I use cut-up pool noodles but same idea). That way the soil that you put above the filler doesn't sink down.

Or, maybe fill with rocks, or that shipping popcorn with rocks on top.

bkay

I think I’d use a cache pot as big as will fit. Lovely urn.

What size is your root mass? In general, a pot should have room for the root mass to grow, but not over 3" of room for it with hostas. As suggested for optimal happiness of container grown hosta.

As someone that worked at a nursery that did a lot of hostas.... a 1 gallon nursery pot might be fine for a year or two of growing a smaller or younger hosta. By the time a lot of average sized hostas are a couple years old they should go into 2-3 gallon pots. I had a joy of breaking up 5 gallon pots for a few weeks one spring :)

So, if your current root mass is a fresher planting and has room to grow, but not too much. You are fine for now. Perhaps consider some bark chip or moss type for pot filler between your inner and outer pots. You can even stuff in the space with plastic bags and top off with something pretty if you want. Don't use soil for your filler. It will indeed drift. And be right unhandy if you need to unpack your pots.

If your root mass is less than an inch or so away from current pot walls, I would transplant into a bigger pot that fills the space better and allows the hosta it's room. If your plant has been in that pot for a couple years, or you just don't know because it came from a nursery and you don't know when they last potted up... pull it out and check it.

Beesneeds, I passed on your suggestion for filling with mulch to a lady at the garden center. She was very pleased and said thank you several times. Thought I'd let you know.

bkay