I ground off the old, dead, Volvo speakers from the frame with the Dremel and a little cutting wheel (as shown partially by the yellow lines).
Then I filed the resulting hole so it was flat with respect to the back of the plate. (no pic)
Next, I fitted new 4 inch, 4 ohm 20W RMS Kenwood speakers to the frames.
This allowed the OE speaker grille to be used and it looks completely OE.
Total cost? About 2 hours faffing and $50.
Couldn't see much point getting higher powered, more expensive speakers in my case, since 20W RMS was more than the OE Volvo radio was capable of.