It's kind of a hard question to answer since the amount of bass that different people would find acceptable is so subjective. You could try adding just door speakers and see how you feel, but frankly the hu in the Z just doesn't have enough oomph to drive aftermarket speakers with any justice whatsoever.
If you really don't want to add a sub and box to your car you could consider adding just a small amp to drive some aftermarket door speakers more efficiently. You wouldn't be getting a lot of low end, but I have heard component sets that are more than competent with midbass response and sound wonderful without the additional weight of a sub, assuming that they are powered appropriately.
There are pretty efficient amplifiers out there nowadays that are designed to accomplish what you envision. I would imagine that you could add door speakers and an amp for a total weight cost significantly under 20 pounds (maybe even under 10). Unless you are a track guy obsessed with decreasing weight this is pretty much negligible. Hell, my laptop bag alone probably weighs 20 lbs.
I purposely didn't give speaker or amp recommendations to avoid jacking your thread. We have more than enough threads in the AV section discussing the merits of various speakers and amplifiers, and we can sometimes get a little...passionate. Even still, I hope some of these general thoughts possibly give you some ideas to contemplate. Best of luck!
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