I'm a big fan of sleeping in pitch black. When I was in college I used to build these light-tight caves around the bunk beds I slept in and I've never slept better. Since it's a bit strange to build blanket caves in the real world, I started to use a little sleep mask. (I thought they were ridiculous until I tried them, they're totally worth it.) :)
I used two pieces of black fabric to make my sleep mask-- one of satin for the front and a piece of a cotton t-shirt for the back. I sewed a cat shape to hold the two together and left a little hole open to flip it inside out.
After I flipped the fabric right-side out, I put my old, boring sleep mask inside and stitched up the hole. (You can just use stuffing if you want.) Then I ironed it flat.
Stitch a little cat nose to it and sew on the elastic that you've measured against your head. That's it!