The British 2p is my base of choice.
I find the Queen has a large, flat face so it's best to use that to stick the model to.
Ireland had 1p and 2p pieces before the Euro changeover. They were exactly the same size as the UK equivalent. The tiny little ridges on the harp strings made an excellent sport for adhesives!
I still use 1p or 2p pieces when I can get them. The Euro 2 cent coin has pretty much the same dimensions as the UK 1p piece.