Ant,
Looks effective! Your interior is very clean and smart, must be quite cheering to get into that in the mornings.
I personally prefer the sound to come from the front, but you can't beat your install for quickness and bass-for-the-money! Don't forget to grease those tailgate hinges - mine have almost worn out again.
Stereo installation in the Mk1 was something of a joke

FIAT provided ONE wire - the brown, live-at-all-times power cable. And the aerial, sometimes. The rest is up to the installer!
FIAT suggested, in the handbook, to use the rear shelf support locations (similar to the Mk2), and the front door pockets (bad luck if you had a basic Uno with the pocket only on one side!). I think there was also some comic illustration of a mono speaker attached under the glovebox.
I suppose that would be a step up from the car stereo (oops, err, 'car mono') in a friend's 1995 Ford Festiva (think 'Kia' from Korea) that had a factory mono AM radio with a 2x3cm speaker built into the head unit itself! 'Sound-quality' wasn't the term. Apart from the feeling of listening to half of a headphone buried in the dash, the only AM stations seem to be horse racing commentary or religious/evangelical... personally (and with no air-conditioning and the 'performance' of a 1.3L 3-speed auto) I'd rather take my scooter to work - which I do anyway
The thing is, in the Uno, only the rear shelf supports were actually ready to accept mounting screws! The door pockets had strange 'slots' (like on the bumper corners but in miniature) so you would need something like square nuts to slide into the notches, and matching machine screws of the correct length. And this is assuming you had a 125mm speaker, not really a common size (5"?) After all that, the back of the speaker would be exposed - which is NOT the way to get good sound (speaker should be in an 'infinite baffle' which basically means a closed box) - and you'd have to run the speaker wire into the door yourself, which means taking the trim off. May as well fit the speaker into the door trim panel while you're there!
Meanwhile under the Mk1 glovebox, you discover two semicircular notches, also not very 'screw-friendly' and the location is in an awful place for sound anyway.
Even the head unit location shakes like a leaf and rattles like a baby... so all-in-all, quite a challenge!
One of the best Uno stereo installs I've seen comprised two or three home hi-fi speakers, in the large old wood-effect dusty cabinets having the torn cloth fronts in faded black/brown/yellow, loaded into the back with the rear seat folded. The speakers were as long as the space was wide. Somehow, four occupants still fitted into the car - maybe sitting on the speakers, I can't remember that bit! Maybe the speakers were 'stacked' behind the rear seat when necessary. The thin grey speaker wires trailed across the floor, wrapping around the gear lever before passing into the stereo/cassette, of the classic two-shaft type.
I've also seen various combinations of stereo-in-dash, stereo-on-floor, speaker-on-floor, speaker-in-glovebox...
-Alex