I have two basses with identical control cavities and lost the cover for one. I got a sheet of ABS https://www.ebay.co.uk/itm/262093434727 and used masking tape to roughly follow the lines of the edge of the existing cover, then drew round the cover on the masking tape. I scored along the straight edges and broke the ABS to give a rough shape, then used sandpaper to get it properly to shape. Drilled and countersank holes for the screws, and all was well.