If you are intent on getting your URL's to be static, then there is really only one module that handles this safely and effectively and that is CEON's URI Re-writer module.

Bear in mind that NONE of these modules has any positive or beneficial SEO value in zencart's case.

Your URL's as "understandable by humans" are useful if you intend to post these on external sites and blogs, where it makes sense for them to be "understandable".

Search engines will index, log and rank your site just fine without them. In fact, SE's generally prefer that the URL is not "tampered" with.