I could suggest a brand and series of amps that would easily fit in the available space. But I don't own a mk3, so I wont.
To the op, just because someone doesn't list owning a car as new as yours doesn't mean their knowledge is less valueable.
I personally own two mk1's and have friends that span all three gens. I have worked on all 3. Does the fact that I didn't buy one mean I don't have knowledge?