For each sheet you will need:

4" x 4" Aluminum foil.
1 1/4" x 1 3/4" cardboard
tacky glue

Fold the aluminum foil, shiny side up, around the cardboard. Make sure you cut away any excess folds at the back. When it fits to your satisfaction, take a round object like a pen and roll across the top to flatten out any wrinkles. Do the same to the back. When it is perfect, put a dab of glue under the folds at the back, and press down again.