Guy, There are people on ebay who sell custom cut sheets of 3mm acrylic/perspex. A 1000mm x 500mm sheet for about 17 quid as an example. A builder of your talent could knock one up pretty quickly.
Thanks, its not that easy in the UK, people don't buy stuff like this very often so its a proper mission to find a good place
Robert that reminds me Ive still to check that dim for you on the 312T4 part, sorry will get to it over the weekend