How do I make the plastic rear-panel snap into the aluminium top?

Yeah, hate the gaposis thing.

The usual way to open a Mini is with a cheap, thin, plastic putty knife.

You flip the Mini upside down onto a clean soft surface. I use a corrugated plastic roadside sign.

After pressing the putty knife into the gap between the base and left and right sides, you should hear horrific cracking and popping.

And this is where the problem lies.

You are bending plastic clips to release their grab on the case.

Check the clips for signs of strain. Sometimes you can shield the innards of the Mini with cardboard and heat the clips, especially towards the back end, with a 1500 watt hair dryer.

After they are hot, shim them slightly outward with cardboard and let them cool.

Then reassemble. This fixes the gap about one in three times.