You can also shoot the piece nestled between two photos of completely
black backgrounds. Here are the steps:
1) Create a completely black backround in Photoshop (or other
software) and print out two copies on glossy photo paper. Depending
on the size of the piece you are shooting, you may want smaller or
larger black photos.
2) Staple the two photos together at one end like a book, with the
black sides facing in towards each other.
3) Pin or tape the back of one of the photos to the bottom of your
photo box, such that the stapled end is towards the back.
4) Attach a piece of string (with a needle or tape attached to the
end of it) to the back of of the top photo such that you can stick it
through or on to the inside top of your photo box.
5) Place your piece sandwiched in between the two glossy black
surfaces and adjust the height by lifting or lowering the string
attached to the top photo. You may need to experiment with this...