You know what though, being a stationary bed REALLY helps you out here.
I'd be inclined to look at the technologies used for radiant floor heating. A coil of PEX tubing, laid out in a maze shape on forms and with a pour of fine self-leveling concrete to make a slab, then that surface polished. Heating could be one of those mini (~2-gallon?) electric point-of-use water heaters designed to mount under a sink. You would need a pump to circulate the water, and some valves and such since it would be a closed system. The bed temp would be whatever you set the water heater to.
Man... 5x5'... You would probably have to have TWO bed surfaces, and let this slab float/slide on top of another surface so it doesn't walk itself away during the heat/cool expand/contract cycles... sort of the opposite of frost heave.
I bet you could get a 5x5 piece of glass though for the actual print surface. It's a bit large to find a glass table top (like a coffee table), but I bet a window supply shop could get a sheet that size.
Looking forward to what you come up with.