Why not make or have made for you various prints or transparencies of a natural suject matter? I would suggest that something like Arista Lith filmwhich is reasonalble in price and can be nothing more than a projection made from a negative with the latent image bleached in Potassium Ferricyanide would allow you to create a number of backgrounds transpaerncies that would have no depth to them. If you are working in color then you could use a color print or a Duraflex transparency for a background. The use of Polaroid foils and a Polarizer on your camera would give easy control of reflections at the loss of 4 f stops.