its a bit hard to judge size. if theyre a good size id say go for it or u may find some milk crates that places are getting rid of. i use those as nest boxes
The round one looks like a small egg might fall through. Would be tuff to keep some bedding in there and possibly hard on the chickens feet. I think they would prefer a flat solid bottom.
If you look through these you can see some of the things that people have used for nests. You'll see that your thought processes fit right in with some people.
However @mowin raises good points. Can an egg fall through those holes? Will it hold bedding? To stick with the theme I'd think straw or hay might look best for bedding but if you have it Spanish moss might look pretty good. Remember you are mostly limited by your imagination.
If the holes are too big you might consider covering them. Some people like to put feed bags or something like that in their nests. Some use feed bags as bedding, others put them on the bottom so the bedding can all be lifted out to make cleaning really easy.
You do not want a chicken to land on the edge and flip the nest over so attach it so it will remain stable. That might mean a frame to drop it into or attach it with a staple or some type of clamp. Whimsical is one thing, impractical is another.