A good auto trim shop should be able to make a set of mats for you. They take automotive carpet,sew it to a rubber mat for backing, then bind the edges together. You can even get the carpet embroidered with a VX logo. The downside is the expense. Any custom work like this is likely to cost hundreds of dollars. The chance that some aftermarket MFGs make a back carpet for the few people who remove their seat is unlikely.
I have removed my back seat [and am trying to sell it] and am contemplating making a mat set as described. I have removed the seatbelts and all seat related hardware. I am also looking at boxing in the rear foot well area and creating a hidden storage while leveling out the floor. I have many projects going for my VX at present and when and if I ever do this I post pix.