If you want rear speakers, but don't want them to be visible, I took advantage of those convenient holes and added a couple of 4" round speakers under a custom package tray I made. I too had to remove a couple of 9" ugly speakers someone had put into the car.
Unfortunately I never took any pics of what I did, but in a nut shell, I got a piece of thin hardboard (about 3/16" thick) measured and cut to size. I then marked the position of the cut outs in the steel panel on the hardboard. Using some 1/2" plywood, cut to the shape of the 4" speaker and cut out the center. This forms a ring that the speakers can be screwed to. On the hardboard, I drilled a serries of 1'4" holes in a circle the size of the speaker cone. Screw the 1/2" rings to the hardboard from the top side, then on the back side and screw the speakers to the rings. I covered the board with some black felt like carpet I found at a local automotive store. You can use anything that suits you, but it ought to allow sound through without distortion. The result is a nice clean package tray and for me, plenty of sound. I only use an am/fm radio as I much prefer the sound of the pipes.
In the pic below, you can just see the "carpet" I used. Sorry, best pic I have of it.
