I picked up a piece of cheapo carpet (the thin kind we put in apartments) at
Home Depot for my '84, cut it to fit about a quarter inch under the rear
seat and fridge etc, and it works fine. The color was almost a perfect match.

The hard part is getting it to stay in place. After trying light spray
adhesive, then velcro, I ended up putting two small stainless steel screws
into the wood track-fill pieces, whatever you call them, that run sideways
across the floor. The screws and a piece of L-shaped aluminum at the door as
a threshold do the job fine.

The carpet is cheap enough ($4 a yard +-) that when it gets really dirty,
you can just replace it.