How good do you want it to look? Just make sure the corners and the edge are cut smoothly.
What are you expecting from a screen protector? Even if 3rd party makes one, it will come flat when they ship it to you. The Apple watch screen is curved on the corner. One way or another, you will need to use a hair dryer to get the corners to fit. It's the same technique used on car tints.
I've used a similar material for the back of the iPad. So instead of having a full case, I can use the iPad bare and when I put it down, the "screen protector" material protects the aluminum. The corner needs a bit of hair dryer work to get the curves to stick.
Here's what you're asking for:
http://www.techarmor.com/tech-armor...t-screen-protector-retail-packaging-4588.html
It's the same thing. They're selling at the same price as the iPhone ones but you're getting a smaller screen protector. Also they went the lazy route by not making it cover the whole screen. You wouldn't need to use the hair dryer, but the corner of the screen will be scratched.