I don't have one but know that the spring on them have a problem. They break a lot and it takes a long time to get one. I don't know if that issue has improved. Flip Packs are a good idea though and I think they still make them. I have had a Wildernest for 30 years and have gotten a lot of use out of it. They stopped making them around the mid 90's which makes obtaining parts an issue. Fortunately, I have take very good care of it and it is in great condition.
Good Luck in your search for a Flip Pack. They are out there. Take a look at Expedition Portal's website. They pop up on there occasionally.