Said and done. I took out an open frame I’ve been meaning to give a makeover (it was a white shabby-chic thing), and painted it black. I covered a piece of thin cardboard taken from a corn flakes carton with a scrap of green velvet from my stash, and embroidered “Muggle Money” on it, through the cardboard and everything for stability.
Then I played with the coins until they made a nice composition, and glued them into place. I finished with gluing the velvet covered cardboard to the back of the frame (not using too much glue, in case I want to give it another makeover in future), and that was that.
It will make a nice addition to the decorations of my now annual Harry Potter Halloween party. I'm sure Arthur Weasley would approve.