Had a friend who had that issue with cup holders in a rear seat (not an i3) and her dog stepping in the cup holes. What she did was buy a fairly heavy rubber backed door mat at a discount place (Ikea, or Tuesday Morning, don't remember which), that didn't clash with the upholstery. She cut a piece from the mat that exactly fit over the cup holder section. Then she took some sturdy soft plastic kids lidded drink cups from one of the gas stations, which fit the cup holders exactly (I think she had to cut an inch or so off the bottom of the cups to get them to sit close to flush. She glued the lids to the cups, when dry she dropped the cups in the cup holders, put glue on the lids (car trim adhesive from an auto parts store), placed the piece from the door mat on top of the cups, put some weight on it until the glue dried, and she had a nice removable cup holder cover, that her dog could step on and not end up stuck in the cup holder holes.. I think she ended up spraying it with some fabric paint to better match her upholstery or trim.