get a sheet of12mm thick (minimum) MDF board from your local DIY shop, with lots of no more nails or other wood glue and some nice big clamps stick this to the underside of your existing parcel shelf.
cutt appropriate holes through shelf and board with a jigsaw, fit speakers ( making sure that fixing screws or botle go through both shelf and MDF), cover with accoustic cloth ( can be bought for about a tenner from ebay in a number of coulours) I find a strong staple gun helps for this bit..
Whole process takes no longer than a couple of hours and you can't see the speakers anymore.