This, I must admit, is adapted slightly from a tutorial on a site I can no longer locate, but as my first major project I thought it might be worth posting.
You will require two differend shades of Fimo or your preferred polymer clay - one for the base of the tentacle, and one for the suckers.
I made this for my fiance, as his Valentine's gift this year - I can recommend giving it only to people who know you very well. I must apologise for the blurry images; they were taken on an unfamiliar camera so the focus isn't amazing. I had perhaps been reading a little much Lovecraft before I made this; you see what you think:
Cut off a little clay from your base-colour block. Warm it up between your hands and roll it out on a board to form a conical length - that is, wider at one end and tapering to a blunt point at the other. Don't use too much clay, or the tentacles will become too heavy to bear their own weight.
When you have four tentacles, or as many as you feel your box suits, position them on the box according to how you think looks best. It may be possible to wire them internally so they hold their shape without needing to rest on something. You will need to squish the wide parts of the tentacles against the wood of the inside of the box to provide an area for the glue to hold.
Roll out a quantity of your second colour of clay. Cut small amounts off and roll them into balls. With your paintbrush, pick up a ball and push it against the tentacle. Pressing hard will result in a dent in the middle of the ball; this is exactly what we want to create the effect of suckers along the tentacle. Graduate the size of the suckers to match the width of the tentacle. If you've very steady hands you may be able to create two rows - I only managed a single line. This is the point you may begin to wish you'd just bought your beloved a DVD or something instead.
When these are completed, this is pretty much how your box should look. Now you're ready to bake it, to set the clay. It may be useful to leave the project overnight in the open air to allow the clay to harden slightly, the better to keep its shape in the oven. Bake your clay according to the instructions on the packet. You won't be able to put the box in the oven, so you may find it useful to drape the tentacles over an upside-down loaf tin or something similar to help them maintain their shape.
While the tentacles are setting and cooling, varnish or stain your box as desired. When the tentacles are cooled completely, give them a coat of fimo glaze. Leave to dry, then give them a second coat. It may be worth using the time to give the box a second coat of varnish, too. Do not attempt the next step until both tentacles and box are entirely dry, according to the varnish manufacturers' instructions.
Once both box and tentacles are completely dry, position your tentacles in their previous positions. They may have changed shape slightly during the baking process; it is unlikely that this will be serious enough to affect their placement on the box. Warm up your glue gun and place a drop of glue on the underside of each tentacle, on the part you squished down to grip the box. Leave to cool, and this should be your tentacle box finished! Present it to your beloved and watch the confusion spread across their face.