Most people I know who do that (who have mac in dirty environment) use plain old contact paper (like the clear type you used in your youth to cover those paper workbooks with). Most don't leave any residue, but to prevent that, just put a piece facing the other way so that you sandwitch the glue where the keys are, and leave a tiny part sticky and stick it to the back of the keyboard...
... it's also a cheap solution so you can change it often and get a clean one.
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