There's a neat trick used in the 3D Printing hacker community to make a heated bed-plate using a printed circuit board (PCB), with one or more copper traces mapped out over the surface of the PCB as the heating element, and then using the smooth reverse (non-conductor) side as the "working face", so that the PCB acts to even out the surface temperature. For best results, use a sheet of thin cork or similar on the conductor side to retain heat, to ensure the heat flux goes to the hot working face, rather than just radiating away as waste heat.
http://reprap.org/wiki/PCB_Heatbed
There are many variations on this theme - glass or aluminium sheet as the plate, nichrome wire or power resistors as the heating elements, etc. This web site might give you some ideas:
http://reprap.org/wiki/Heated_Bed