[Shop-talk] Rivets backer

[Robert nogueira]

I need to install some 1/4 inch rivets in my current project. These are the
old fashion solid metal rivets with a domed head. I have had no problem
mushrooming the stem with a hammer. My problem is that the round head tends to
also flatten on the crown.  What I need is a backer that the round head can
sit in when hammering the stem.  I'm sure there is a tool for this but I have
no idea what it is called much less where to buy one.
Can anyone help me with a name and source for such a tool?
Thanks for any help.