You could also push a 15mm diameter marble/ball bearing anything along those lines into a chunk of plastercine to required depth.
Push out from the bottom with a bit of 1mm brass wire/rod or needle.
Paint/stipple fibre glass resin in to reduce air bubbles.
Then pore more resin and hardener mix it to the top.
Leave overnight to make sure its solid (or put loads of hardener in to cure faster). Sand to finish.
Paint to required colour. Sounds a lot, but only take about 20-30 minutes plus curring time.