Rear speaker setup recommendations

[JudgeM]

Hi All.

The next project I would like to start on my Vitz is the audio system

I was a bit disappointed after buying him to see there are no speakers in the rear. So as a quick fix I went the home stereo route (see pic 1). This works quite well and I am happy with the sound.

However it looks unprofessional and the speakers do slide around.

For some strange reason the JDM Vitz do not come with rear door speakers. The hole for the speakers exist but no provision in the door card (pic 2)

Can you guys give some recommendation on how I can make a better set up ?
Main thing for me I want clear sound, don't need crazy bass and ear shattering highs.

So far I am thinking of the following:

-I bought door cards from a NZDM Yaris which has rear door speaker provision but it doesn't match my car so I am not keen on using them. I was thinking of cutting holes in my existing door cards for speakers
-I was thinking 6x9 at the rear but I have no parcel tray and I don't see an easy to add one.
-I am also thinking to get a small bass tube or one of those slim all-in-one woofer systems.