I have. In 2 pieces and stitched around the sides and bottom. I take it the impala has the hair? If not you can wet and mold the leather to fit but if it has the hair I'm not sure. You can still stitch and just keep pulling the little puckers tight and take your time. This one was a temp trial.
You can do a better stitching than I did, ill do a nice x stitck when i do it permanant. Now that I have the fit I'll just take this one off when I find a piece of leather I like and get around to it I'll remove this one and use it as a template.
I would reccomended doing the same before cutting your impala.