Could do that.you could easily cover the whole parcel shelf,
you may well loose some high end sound,
if you stretched it over the whole shelf, and not push it into the recess in the middle,
it will cover them nicely,
id use as thin material as possible, as the thicker cover you use, the more noticeable it will be in the sound quality,
something like the marterial tights are made out of would be idea.
Did exactly what you said, covered the whole shelf in thin carpet from the local audio shop, i was amazed how easy it is to do, especially the spray adhesive which i was very impressed with.spray adhesive,
take the speaker out, spray the top of the parcel shelf,
(cover up the reccess so it doesnt stick there)
put the speakers in
and lay the carpet over the top,
or if you want to be able to remove them easily,
you could screw them in from underneith, but that may lead to them falling out over bumps