Richard - I have built and use a router constructed in MDF. Can't remember the exact cutting size but it's less than 1000x600. It bends when it gets warm, it bends when it gets cold, when it's dry, when it's humid, when it starts cutting and when you look at it. It also warps when you are not looking at it... Cold-rolled cow dung is not a sound structural medium. Probably not so bad if you paint or varnish it well and keep it in a warm dry environment but I'm about to start on a steel-framed replacement. Don't know about Corian although it's probably a lot more stable but I doubt if it is really fit for a structure of the size you are proposing. For sign-writing type use you probably don't need enormous structural rigidity but if you can't keep the bed flat, you are not going to get uniform depth lettering.