Once upon a time i was looking for a printing platform for my printrbot simple after I had upgraded its print area. This lead me down the usual rabbit hole for a flat and available surface to print on. Things tried: This leads me to my current build plate, its a 200x300mm by 2mm thick carbon fiber board. Carbon fiber is very strong so it will hold its shape. When I ordered a board form ebay I did not know if it would be flat enough to print on. Upon examination the board is very very flat. It also has a nice shinny and glass like surface thanks to the resin that the fiber is bonded in. When i try to bend the board in my hand it flexes very little. Other things to note about the carbon fiber, it was cheap from china. There are some defects in the layout of the fiber but that does not affect the smoothness of the board at all. I wouldn't go thinner than 2mm thick and if the print surface needs to be bigger I would recommend a 3mm thick sheet for a comfortable level of rigidity. This is my first week with the carbon fiber bed so I will post any updates that will find! UPDATE 2/4/2015: No problems with the build plate. I use glue sticks as an adhesive. I get a lot less curling problems than with the glass beds I used. As long as there is glue down the print will stick and will be flat. I have sense printed a large object on the bed with no problems, 250mm long. Find a carbon fiber sheet on ebay that is the right size. Clamp it to your heated bed. (or non heated) Apply coating of glue stick for optimal adhesion. I have yet to try out anything but PLA but the surface acts like glass so an ABS slurry should work well.