Print it on an aluminium frame and fix it to the wall using screw(s). This makes the print flat even when the size is bigger than shown in your picture. Nice beechwood, by the way.
Prints on aluminium can be had with a supporting rear rectangular frame of metal tubing, which makes mounting easy. That is my preferred approach for large prints. Ensure the print is coated with UV-absorbing spray so the colours don't fade too much over time.
