I found this porthole on Ebay for $60. It was a little steep, but I figured my odds of finding one in a store or antique shop were pretty low. It's a functional porthole, too - the little glass window swings open, which I thought was fun. But once it arrived and I held it up to the wall, it felt boring. The color clashed with the wall color and it just seemed a little bland, so I decided to jazz it up!
I purchased Chalkboard Paint and a Textured brown spray paint from The Home Depot. With the Chalkboard paint, I sprayed 4 light coats directly onto the glass window. I was worried it wouldn't adhere to glass, but it did just fine. The key was spraying it lightly to avoid drips.
I then sprayed the entire metal frame with the brown textured paint. It really gave it that old, rusty look. The less "perfect" you spray, the better.
Then, the fun part - drawing your monster! I used a chalk marker to make the white pop a little better. What's awesome is that everything you do is erasable, so no need to freak out about any mistakes! I googled some different octopus and sea monster images and combined a bunch of elements for my own creation. After I hung it in our bathroom, I'm not even kidding, Joe must have stared at it for a week during bath time! Spooky! :)