I like using Wild Ginger - nice silvery green compact foliage that stays low & won't overwhelm Helleboros. It's edible too.
Christmas Fern is another option that works.

Actually I would give them a container to themselves for two reasons. First they are so beautiful in flower they deserve the stage to themselves and second the leaf spread is so wide after flowering it would be in competition with anything else, and also it gets quite untidy.. I would have a second container of something else to take over when its finished, or to stand nearby if you want other flowers at the same time. All depends on how much space you have in the area where you want to put the container.
Then when the hellebore is over you can just move it to somewhere inconspicuous or 3ven sink the pot into the ground for the rest of the year. I get round the problem by having ordinary plantpots planted up, which I can then put into the container as and when the appropriate time arrives. I use the black flower buckets that Morrisons sometimes sell off very cheaply - just make a few drainage holes in them and away you go.

Good tIp about the flower buckets Stera. I will try your idea of keeping the plants potted and switching them as needed since I'm finding the alternative more difficult these days.

Yes, anything to make life a bit easier! That way you can have daffodils and tulips in succession in the same containers - magic!