Want to be the Belle of the Bacchanal? In this Instructable I'll show you how to make a pair of lightweight custom horns out of open-cell foam. The process is simple, the materials are cheap and easy to come by, and the results are stunning!
You could bang these out in a few hours (plus drying time), and the total materials cost was less than $20!

Step 1: You Will Need:

You should be able to find all of these materials at a well-stocked craft or fabric store. I found a big chunk of foam on sale in the upholstery section at Jo-Ann and made 4 pairs of horns out of it. If you're a frequent crafter, you probably have half this stuff lying around already!

Materials:

Open-cell foam
Scrap Leather
Elastic Headbands*
Paper for Templates
Acrylic Paint
Clear Satin or Gloss Varnish
White Glue

*If your horns are big enough, you may need to use two headbands to keep them from flopping around. If your horns are small enough, you can just attach them with spirit gum instead of a headband. I, however, like being able to put them on and take them off when needed and don't really mind that the headband is visible.

Tools:

Scissors
Hot Glue Gun
Needle & Thread
X-acto or other Craft Knife (sharp!)
Chalk Pencil or Marker (for drawing on leather)
Leather Punch (not necessary but convenient)

Step 2: Rough Cuts

Draw the shape of the horns that you want onto a piece of paper and cut it out. Trace this template onto your pieces of foam so that your horns are mirror images.

Cut the rough shape of your horns out of the foam and then round the edges as shown. I find it easiest to make big, rough cuts with a craft knife and more detailed cuts with scissors.

Step 3: Trimming and Texture

It is IMPOSSIBLE to cut a perfectly smooth shape out of squishy open-cell foam. Seriously. But since we're trying to mimic an organic form, we can use this characteristic to our advantage!

To create a lifelike horn texture, make lots of small cuts with your scissors perpendicular to the curve of the horns. I find that it works best to work on one horn until I'm satisfied with the shape, then use that as a model for the second one. Pro Tip: If you make the base of the horns (the part that presses against your head) slightly concave instead of perfectly flat, they will conform better to the shape of your head and won't flop around as much.

Step 4: Sealing the Foam

This is the part that takes the longest and is the most irritating. Cover your work surface with a plastic bag for easy cleanup.

Using a paintbrush that you don't care about, coat the surface of your horns with white glue (I used tacky glue, but you could use PVA or anything similar). The method that worked best for me was to paint one side and let that dry, then flip them over and paint the other. Pro Tip: If you plan to attach your horns with a headband, don't seal the base of the horns because the hot glue will stick better to the raw foam than it will to a smooth surface. If you plan to use spirit gum, you should seal the base of the horns as well so that the spirit gum won't sink into the foam.

Apply 3-4 coats until you can no longer see pores, just a smooth glossy surface. You can dilute the glue a little with water to make it easier to paint with, but then you'll have to apply more coats.

Step 5: Creating the Attachments

This is how our horns will attach to the headband. If you're going to use spirit gum, skip to Step 8.

Trace the bottom of your horns onto a piece of paper. Draw 4-6 tabs radiating outward from the edges as shown (this may not be absolutely necessary, but it's sturdier than just gluing to the bottom of the horns). Trace 2 template shapes onto your scrap leather and cut them out using a craft knife.

Step 6: Adding the Headband

Using a chalk pencil or marker, mark on your leather where the headband will thread through. Punch 2 holes in each piece with a leather punch, or cut 2 slits using your craft knife.

Cut the headband open and thread it through both attachment pieces, then sew the ends back together. I hid the seam inside one of the attachment pieces.

Step 7: Demon Horns, Assemble!

Using a hot glue gun, stick the bottom of your horns onto the attachments. Pro Tip: if you don't get any glue on the headband, you will still be able to slide your horns around to adjust the spacing.

Once that glue has cooled, apply more to the leather tabs and press them onto the body of the horns.

Step 8: Decorate!

Time to paint your horns! Since the glue we used is water soluble, it might reactivate a bit when you paint on top. The wetter your paint, the more this will happen. Make sure you allow each coat to dry completely before adding the next one or else you'll get glue spots bleeding through the paint or cracks forming as the paint dries.

So, using UNDILUTED acrylic paint, apply two coats of a base color. Then, to create a realistic sheen, use either a satin top coat or mix roughly two parts acrylic paint with one part gloss varnish.

The rest is up to you! I had fun brushing a contrasting color onto the ridges of the horns and adding metal effects with aluminum tape. You might also want to disguise the headband with beads or fake flowers to make it a little more decorative.

Congratulations! You're now ready to go out and raise some Hell!