I think if it not an official cover, it should not be used. Otherwise, as @climhazard has said, and has already been done with several games, you should list as XXXX (Long Box Flat case) or XXXX (Jewel Case). Others have also done it with certain Sega CD games that had multiple CD designs. I don't know if the slipcover is considered official or not, but I think serious collectors would hate to see such unofficial stuff on their list, just as why I wouldn't list my fake pokemon game (that I am still pissed about to this day).