Here is a very easy way to give Optimus Prime a face plate. Just use the box that the figure comes in and cut the clear plastic edge into a diamond shape until to get the right size. Paint it silver before you glue it on or put it on and then paint the entire figure if you are doing a full repaint. I used this for my http://www.tfw2005.com/boards/radicons-customs/988003-tfp-ultimate-optimus-prime.html and also the voyager Beast Hunter Optimus.
the best ideas are usually the simplest ones. and if it's cheap and easy to do, more kudos to you! thank you for sharing this ingeniously simple mod, Solrac333!