Personally I don't think that you can make 6,5" subs play low frequencies powerful.
At least not as powerful as a 10" or 12".
I'd suggest putting a closed box with a 10" or 12" into your trunk/boot. It doesn't take much space (a closed box can be quite small), is easy removable and is very dynamic.
You could then adjust your amp to filter the low-frequencies from the 6,5" speakers and filter the high frequencies from the sub. This will give a high-quality and powerful sound.

A 10"/12" is about $100 and an enclosure for it is about 50$ so this would not be very cost-intensive.

The canton are very good, I have them (well - not the exact same model) installed in my front doors and in my dash. They give a really clear sound.

That's what I'd suggest. Doesn't mean it's the best solution.

TeeMcBee
Got my Mk2! # 080000143