Try a commercial kitchen supply store (theres one in Newcastle St Northbridge) and look at a large cutting board - I have one 600mm x 450mm mounted on an ironing board - they aren't too expensive in about 12mm thickness and easy to clean up.

I recently bought some material for making up a heavy duty crab/cray gauge like the ones on The Deadliest Catch.

I ended up getting it from a place in Canningvale called DOTMAR. the guys name is Ian and his number is 0410582844.

They have all types of plastics in any size you need. 

 I grabbed a door from a kitchen cabinet/cupboard from the tip that was in good nick.  Took the handle off, filled the holes with putty and as its laminated wont absorb water or fish blood.

50 cents for some stick on rubber dot thingys stop it sliding around on top of my icebox.  When its eventually gets ruined, I'll just go and get another one.

Plenty of different sizes around

just get yourself a piece of pine about 25mm thick and stick it on or just clamp it down. treat it with a bit of veg oil every now and then and it will last for ages. just give it a scrub after each use and you wont have a prob, I have always used timber cutting boards without an issue

 use an offcut of marine ply for the last 6 months and still going strong

 i have a piece of marine carpet glued to mine, it's hard wearing enough to cut on and your fish dont slide around when

filleting

Thanks guyS