Car audio is a very personal and subjective matter.
My choice would be - two 4" speakers in the rear shelf locations (most Unos I see have no speakers or wiring, but the wiring is fairly easy to add behind the rear side carpet trim, then under the boot floor (under the back seat), and finally under the carpet along the sills.)
Then, two 6.5" speakers mounted in the front doors, with suitable sound deadening/sealing of the doors. The Mk2 standard front 4" speakers just aren't good enough and the mounting acoustics are horrible!
I subscribe to the widely-held view that it is better to have a 'sound stage' set up at the front of the car. Most of the sound should be at the front, with the rear speakers only just noticeable.
That said, when you buy cheap speakers, size counts. The quickest way to get lots of sound is going to be the 6x9s that Ucof said, mounted in an MDF rear shelf that you can knock together in an hour or two. I just don't like it myself
Another location for speakers in a 3-dr Uno is in the rear side panels - I experienced that on Jai's white Uno Turbo. Again, I don't like the sound stage, but it worked very effectively - the rear side panels make a good 'box' for the speaker. Slap a pair of 6x9s in there and they will be out of the way (no wiring to disconnect when you need to get the shelf out). Rear side trims are hardly in short supply, so it should be OK to cut big holes in them.
Just some ideas...
"I need to hear the bass from my Speed garage-jazz-funk-soul-fusion mixtapes" - LOL!
I hope you don't actually mean those funny plastic cassette-tape things from the 80s.
My sound system these days is the CD head unit in my 156 that has the optional Bose speaker package, with amplifier and 8" sub boxed into the rear shelf. There are speakers in both the front and rear doors (bit odd, having rear speakers at the front of the rear doors, not really in accordance with the soundstage-thing I mentioned, I think it must be so that the rear passengers enjoy a front soundstage too!) The speakers are extremely crisp, clear, and powerful. I added an XCarLink iPod interface that makes my (new) 120GB iPod Classic appear to the headunit as a CD changer, with discs being playlists and Disc 1 being a direct-control mode. This allows me to dial up Chris Isaak's 'Blue Hotel' on the iPod and then use the steering wheel controls to adjust the volume or go back to the start of the track again and again even though there are about 8000 other tracks to choose from
Ironically you see, the sound system I use now is a standard factory setup. This after about 20 custom installs over the years...

But with the Uno, you get nothing (or very little) as standard, so lots to play with in aftermarket possibilities.
-Alex