I discovered this project in a blog by "2A3 Maniac" Its all in German but you really do get all the info you need from the illustrations. Considering the cost it sounds surprisingly good. 5/8" plywood is closest to the 16mm stuff specified. Have fun all you audio DIY cheapskates
2a3-maniac.blogspot.de/search/label/DIY%20Transmissionline
Hope the URL works, if not you can always google 2a3 maniac transmissionline
Robert Lees