Autocraft Engineering have made a new speaker blanking plate for the dashboard on oval window beetles. These are made to the same standard as the original part and have been test fitted on their own vehicles. They are available from the Autocraft website at £29 each including VAT. 

http://www.autocrafteng.co.uk/shop/beetle_63/panels-and-body-parts_80/front-panels_359/speaker-blank-type-1-53-57_4793/