Suggested method of production.
Make your design out of air drying clay, allow to cure, seal with a waterproof pva. Make a mould out from latex, reinforce the mould with poor mans fibreglass. Fill the mould with concrete to make your plaque, while the concrete is still wet, embed fixing hook for hanging on the wall later. Put the plaque in a plastic bag to slowly cure. Leave for a few days to cure before removing from the bag. Hang on the wall in the garden.