Simo's inherited 306 HDi

[306gtiparts]

I've fitted stumpy self tapping screws through the 4 holes in the glass with foam washers.

Sounds like you have the smaller phase 2 spoiler.  The phase 3 one is a bit deeper and just has two bolts through the metal above the window.

No you can fit a p3 spoiler to a p2 using stumpy self tappers with foam washers in the original p2 holes in the glass. Holds much better and your not left with 4 holes in the glass.