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To copy Boo's image on a piece of MDF, I made a 5 x 5 grid in Illustrator and placed a picture of Boo behind it (the pdf document is included with the pictures). The grid is easy to draw on the MDF sheet, and makes copying Boo's image a lot easier.
I used a jigsaw to cut out Boo's silhouette, his eyes and mouth. Boo's teeth are made out of a separate piece of MDF.
Sanding and painting takes patience (! of which I have only a limited supply :-s). I used two layers of white acrylic primer. Boo's final white is high gloss acrylic paint. The tongue and eyebrows are also acrylic paint, but matte. For the eyebrows, the matte paint is perfect. The tongue's red might be better in high gloss. Use a roller (or even better, an airbrush) for the white and red. I tried the tongue with a wide brush, but with very bad results (brushstrokes stayed visible, no matter how hard I tried...)
To get the red light behind Boo's eyes and mouth, I made an inclusion on the backside with left over pieces of MDF. The inclusion is eventually covered with 5mm plywood, painted black on the inside.
Inside the inclusion I traced the circuit with 3M copper tape. Expensive but very, very useful in this case. I only used it because I had it lying around. I used ductape to make "bridges" on spots where the traces cross each other.
Since 3M copper tape can be soldered, mounting the leds was easy. Again, I used tape to prevent the LED's leads making contact with a reel they shouldn't be making contact with (preventing shorts).
The red glow is produced by a string of the cheapest 5mm red LEDs. Between the leds and the batteries is a circuit I'm not very confident about, it still needs some debugging. So, only very limited details in the picture :-)