Mark. An anvil is simply something you use to push against. I use a piece of metal like a hunk of brass, aluminum, steel, whatever is close.
A proper anvil would be shaped to accept the other side of the rivet, typically a small dome like on a coupler rivet.
Google Brakemans Riveting tool or Paul’s (PE Design) tools to see what a proper tool looks like.
https://pedesignmanuf.com/?page_id=694
Pete