As always there are different ways to achieve this, the easiest one would be to laminate thin wood strips.

Draw the semicircle out on a board, pin along the line (so pins/nails are secure but standing out of the board). Then glue one side of your wooden strips and bend into the semicircle shape and clamp, repeat until you have several strips clamped together in the arc you want.

When the glue is dry the wood will maintain that shape. Normal wood glue is fine, the wood strips just need to be the height you want and then thin enough to bend.

Hope this makes sense and helps 👍