My suggestion is to replace the stock paper cabin air filter with the same size K&N engine air filter. The K&N engine air filters have a thin metal screen built in which should help keep mice from damaging the filter material.
Don't try to use a K&N engine air filter for a Boxster because the cabin and engine air filters are different sizes.
Instead pull the stock cabin air filter and measure it. Then use this link below to find an equivalent size K&N filter.
http://www.knfilters.com/search/univ2.aspx?ID=S0005
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