Acepac - Saddle Satteltaschenharness - Fahrradtasche im Test

It works much better than other brands I have seen. The system works well and the bag does not rattle at all. This helps keeping the bag in place. The fact that you have an harness also makes it easy to just unload your stuff for the night.
I would buy it again and avoid those brands that have the “bag only” system.
Please bear in mind that you must have at least 15cm space under your saddle. That is, you must be riding with your saddle high or the bag won’t fit. This is the case with most bags, but I found that this one worked with less space than others, which often require 20cm space.