You should only put rocks, decor, and plants from the pet store in your tank, unless sterilized with boiling water, and all kinds oh thing to "fish proof" your custom decor. I think rubber might float. And a thing that size could "choke" a Betta.
I dont recommend it. I would only purchase "fish approved" things from my fish store. I would suggest floating betta mirrors. Betta logs and betta leaves. And maybe letting a female float in their tanks once in a while. Lol. They would love that! Rofl.