I don't know if you tried cling film, but it's easy to put on and easy to take off. You do have to buy it, and I'm not sure if that's what you mean by not wanting a specific product?

We used to use Glass Wax to make frosted window designs at xmas. Maybe that would be DIY enough?

The link below refers to acrylic glaze. Rustoleum makes an acrylic spray on window frosting product, but I haven't used it. I have used a cling film purchased at Home Depot or Lowes and it couldn't have been easier and looked terrific.

You also could hang a stained or leaded glass window inside your existing one for privacy.

Here is a link that might be useful: How to frost windows.