Speakers for screen porch not exposed to rain?

Hi, Thanks for your help. I have a summer house which has a screen porch 11x22. The room is pre wired for the speakers to be up high on the wall at the cieling line.(8ft) This location is totally not exposed to weather but is of course exposed to summer heat and somewhat cold at night(across the lake from Chicago). I want to upgrade my 7 yr old Cambridge outdoor speakers. While shopping I have noticed the large price difference between bookshelves and outdoor speakers, also that there are no used outdoor speakers for sale. As the outdoor speakers are usually not ported, I'm thinking of buying a good used bookshelf and bringing it indoors promptly when the weather starts to change. I'm prepared for a shorter life span if it's not too drastic. Thoughts? John-- Thanks. Johnjwi@sbcglobal.net