Hitler's Festung Orders

Hi all, I'm trying to find out a bit more about Hitler's designation of fortresses, specifically Walcheren in the Netherlands. As far as I can see from Hugh Trevor-Roper's Hitler's War Directives 1939-1945, Hitler first issued orders to make various European ports and cities fortresses on 19 January 1944, although not as one of his war directives (but as another order it seems). Secondary sources I have read state that Walcheren was added to this list on 4 September when Antwerp fell, although this again does not appear to have been one of the official war directives.

Does anyone know where I can find the orders officially designating Walcheren as a fortress, and perhaps a little bit more about what the fortress designation actually meant (War Directive 53 of 8 March does at least contain some information on this).

Many thanks,
Stephen