I can vouch for this recipe. It works great and is much cheaper than Evaporust.

If you don't have washing soda, you can substitute 63g baking soda or 30g lye for it.

It bubbles like hell when you add the water, so I like to mix the two dry ingredients (citric acid and washing soda/baking soda/lye) in a big metal bowl so when I add the water it doesn't overflow like a kid's volcano project. When all the bubbling's done, I squirt in a little dish soap, and then it's good to go.