Another approach

[johnmh]

We found that stock vinyl letter could be applied to pieces of translucent plexiglass and then sprayed with matte black spray paint to make the signs.   The lettersare available in various fonts and sizes in art supply places - we used three packages (two 3" and one 1" - transforming Q's into O's and E's into L's to get enough for all signs)

A photo is shown among the others here:

http://tardisbuilders.com/index.php?topic=2835.msg29840;topicseen#msg29840

After the paint is dry, a heat gun is used sparingly to make the vinyl letter bubble - allowing them to be lifted up with a small knife. 

We used 12v strip LED lights inside a sign box to illuminate the signs.  These lights are available from auto supply stores here - and online as well. 

Plastic is available in various forms here- home bulding supply stores and specialty plastic shops as well.  You can get lucky and find suitably sized scraps in bins at some plastic supply stores.  We used translucent white but you could probably use clear with a light spray of white paint on the inside behind the letters after painting the outside black.