Is your bun a big chewer? You’d just want to watch to make sure he isn’t ingesting too much of it. untreated Sisal rope is perfectly safe, and if Snowy isn’t a big chewer the plush should be fine (as long as it’s not treated with chemicals, but you should be able to tell by smelling it). Lots of people give their buns beds/stuffed buddies, it only becomes a concern if they are actually eating it, as it could block up their stomach, just like eating too much fur. But that’s really just something you’d keep an eye on.
As for the particle board (flake board) I’d keep a good eye on that. Pretty much particleboard is a bunch of wood chips they glue together to make a board. (our chicken coop is made out of it) I would be nervous about him eating it because A) you don’t know for sure what type of wood it is made of and B) the glue. So if Snowy happens to rip the fabric off, just patch it back up.
So if you are aware of everything I mentioned, it should be fine.