The first step is to make some ears from cardboard. You can experiment with shapes using construction paper. It's thin enough to be easy to work with, and still stiff enough to keep it's shape when folded. Once you have a design you like, it can be a template for markign the design on the corrugated cardboard.
In this case the square edges are 4cm long. The tip of the ear is 1 cm (3/8") wide, with 1.5 cm (5/8") on either side..
The construction paper prototype can be used as a template to mark out the design on cardboard. Mark them out with the corrugations running from the tip down the length of the piece with the two wings to the sides.
The dotted lines are cut part way though, leaving the bottom outside layer. These are the fold lines.
Folding the ear results in the bottom and sides meeting along the edges of the cuts. That means there's nothing to glue to. Although hotglue provides some structure itself, it's nice to have a bit more support for the joint. You can do this by cutting a matchstick shaped piece of cardboard to glue inside the corner. That lets each surface (the bottom and side in this case) be glued to this support.
All three concave corners can benefit from a bead of hot glue to enhance the bond as well as provide additional structural support. Allow each bead to cool before adding the next or the heat may cause the entire joint to come apart.
The first step is to fold and glue the ears. You can use the method outlined above.
Glue a small piece of plastic straw to the middle of the base of the flap to be a guide for the actuating wire.
Next glue a servo to the back of each ear as shown: as far down and to the left as possible while still not protruding past the bottom or side of the ear. Consider peeling the foil label from the side of the servo being glued to the ear so that it won't later come off on its own.
Poke a small hole near the end of the flap and place a rivet in it. Put the plate on the rivet, but only to the first position. You'll tighten it later. Make a similar hole near the middle of the back of the base.
Place a rubber band around the flap rivet as shown. Use a rubber band that is taunt when the flap is extended and has enough strength to pull the flap back when it is released. Poke the other end of the rubber band through the hole in the base so that 2 mm (1/8") sticks through, then push the rivet through it from the top. Place the plate on the rivet and push it on to the second notch.
Connect the servos to a board and set them to 0 degrees. Put a "half" horn (see the photos to the left) onto the shaft so that it faces up.
Next you need to link the horn to the ear flap. You can use a short piece of wire looped around the rivet at the end of the flap and through the straw guide. The ends can be soldered to a small hook made from a pin from a male header strip bent around the tip of needle-nosed pliers.
To connect the hook to the servo horn, make a loop from another, longer header pin and place it through the outermost hole in the horn. Squeeze the loop closed. Hook the hook on the wire through the loop on the horn and squeeze it closed. That will keep it from slipping out when the servo horn point up (and allowing the connecting wire to go slack).
The goal is that when the servo is set to 90 degrees the flap is pulled vertical. When the servo goes back to zero degrees, the rubber band pulls the flap back down into the relaxed position.
Now you have working ears. Let's make them look a bit better. You can use any sort of fabric to cover them, felt on the inside and fur on the back would be awesome, but here we just use some felt to keep it easy to see what's going on.
Start by cutting an outside piece for each ear. The exact shape isn't overly important; it has to be big enough to accommodate the servo and the motion of the flap, but not so big as to completely obscure the overall ear shape. Secure the corners as shown to the flap rivet, and the front corners of the base.
Cut a similar piece for the inside of the ear. It only has to allow for the flap movement. Tack the corners to the same points as the outside fabric, only on the inside of the ear. Then apply glue along the inside edges of the outside piece, and secure the edges of the inside piece. The final two photos show the result.
All that's left is to mount the two ears, the Circuit Playground Express, and the battery to a headband. Hot glue works well with the ears, while double sided tape will work for the board and battery.