I took the flat padded shelf insert (about 3 inch thick) and opened it up, put in MDF enclosers for the new speakers, re-wired with bullet connectors and re covered in black leather. Less intrusive then the vertical speaker mounts but to be honest, i like those mounts. Here is a picture of standard after market insert from the 70's and still available "new" on web (expensive)
I have bought a set like these from California and intend to redo them with modern speakers if possible...but they are still in CA until i travel there for business and pick them up
My car is in shop and i will send you photo of the flat setup if you wish when i get car back
To be honest, i like your setup..i would look at how the speakers are mounted and alter to frame with mdf to improve sound if possible
Bob