I used an old jewelry gift box cover and put a clay scene on top^^ It's my avatar, Saru-Hime, sitting by a well and trying to fish with a banana as bait^^ It was a lot of fun to make and I hope you like it^^
I was worried about it burning as well so I researched and found this on yahoo answers:
""Paper (and paper products like cardboard, etc.) don't burn until the famous temperature of 451 Fahrenheit. That means the *air* in an oven reaching that 451 temperature though... if you touch the paper/cardboard to the *sides or coils* inside an oven or the coils of a stovetop, etc., the temperature will actually be much higher than 451. If you even get the paper really close to those places, the temp will be higher too.
But as long as you just put the paper or cardboard/cardstock/wood/etc. on the rack (middle rack would be best) of an oven or on a baking sheet then on the rack, and also preheat the oven before putting the cardboard in, there should be no problem at all. (We bake polymer clay on paper and/or cardstock products all the time... usually at about 275 or a bit higher... and often use the same piece over and over.)
Over time, the paper could start to brown a bit from repeated heatings (and because it's touching the metal rack), but that shouldn't be a problem for you.""
Since most polymer clays bake at lower temps, it will be fine^^ Hope that helps^^!