My wife wanted a system that allowed the oldest milk to be used first. We have been filling other containers, but you still have to rifle through them to look at all the dates. There are similar units for sale for $30 each.

It was designed for a 6oz Lansinoh bag of milk, which I calculated to be around 120mm x 90mm x 20 mm. If it could work for other bags, but I don't know.

Feel free to edit the STEP file, just reupload as a remix.