I used to make Willie style straps years ago. It's easy. Just get some red white and blue nylon macrame cord. Drive some finishing nails into a narrow board (long enough to reach from the back of your neck to the bottom of your guitar) staggered left and right every two inches or so and zig zag a blue cord from nail to nail. Tie it securely on each end. Then run the red white and blue cords straight from top to bottom and do a half hitch whenever they cross the blue zig zag cord. Pull the inside cords tighter to round off the corners of the zig zag as you go.

You'll be doing this twice. There are two sides to the strap and they tie together behind the neck.   

Leave long loose ends to secure to a ring at the bottom and enough at the top to knot with the other side behind your neck. Of course you can decorate the loose ends on top with beads.

Then run another cord through the hook and tie the ends to the rings.

You can buy a proper classical sling at online stores that specialize in classical guitar. But the coat hanger trick would work just fine too.