I think a tool used by woodworkers (myself included) to countersink finishing nails would work great for that project. They are made of metal, usually with a rubber grip on the handle end, and the other end has a concave point. They come in a few different sizes, so what size you need would be based on the size of the pin head of your pins, I would think the smallest one. They are available in the tool department at any home improvement store such as Lowes or Home Depot and I think even Walmart has them in the paint and hardware section. Just ask for a "countersink" tool.