Is there a more natural product to use for bowel prep before a sigmoidoscopy?

I've read that the Fleet enema kit ( or any of their products )is not the best and safest way to "cleanse" before a GI procedure.

I'm actually afraid to use this product but my doctor ordered me to use an enema after dinner in preparation. I then spoke to the nurse and she mentioned that I have to take a SECOND enema the next morning ( 1 hour before the procedure) If I consume a light liquid diet all day and fast for the rest of the evening do I really need to take the second enema the next morning? I'm really confused.