My first option I can think of for this, is to add 2 after market 5.25 speakers in a location that is out of the way (under the seats or tucked behind the seats). This involves less work.
Another option which involves more work would be to cut into the back door side panels and mount two shallow 5.25's.. I've done this before on older model Peugeot and it looks like it was factory installed! However, depending on the depth of the side panels this can be a problem and more extensive work would have to be done.
This should be a standard speaker option when purchasing the Peugeot 108.