You can get wooden ones covered in vinyl. I have a set holding my 6x9's in the parcel shelf.
BUT, with a gas tank in the back, I doubt a pre-made spacer will help. I had 6x9's in the back of my old 85' XF and that was on gas. I ended up making my own spacers to put them in, you have to angle them otherwise they won't go in because of the back window.
Either that, or get some fibreglassing done, something that will just sit on top of your mdf parcel shelf.
87' XF FREAK!