One resource that I have been using for years (like 40+ yrs) is a book by Rufus Reid called "the Evolving Bassist". The first section has a 30 pg etude that is just open strings, going from whole notes to all kinds of crazy rhythms. It takes a long time to get through but to this day, I still use it sometimes as a warmup for bowing on DB, or when I feel that I need a refresher course in reading time.