Home made bottom bracket plastic sleeve
Since the only opening through which debris can get into bottom bracket is the seat post (provided it is open from the top), yes, there is a better material than a plastic bottle. Simply stick a wine cork into the seat post blocking the opening. Not only you stop the debris from getting into the bottom bracket, you also stop them from getting into seat post and seat tube.

You can also place a so-called micro adjust seat post, which is closed from the top (picture only for illustration):