Hi Mike, using a good quality waterproof glue and make up a panel the size you need by edge gluing and clamping with smaller matched bits of timber. I would use over sized kiln dried European or American Oak. Do a surface level on both the back and front surfaces to make sure the panel is nice and flat before doing the final job.

Select some nice matching gain and you'll just edge gluing. You might want to increase the depth above the final finished from 10mm to around 15mm plus, DO NOT use biscuits or dowels as these might show up in the relief carving. As long as the finished job is well sealed it should last for many years. Search on YouTube as there's lots of helpful videos on gluing up panels. 4 Tips for Gluing Clamping a Tabletop - YouTube just miss out the biscuits. Mike :)