Much and more has been written already about the Lengberg Castle "bras." I'm not going to rehash any of that except to say it seems fairly certain that by at least the 15th century wearing some sort of supportive undergarment *other than* a fitted gown was not unknown. I've been playing around with making one of these for the better part of a year now and here are some thoughts. I've set up a pintrest board on period undies and another on modern undies to collect images and helpful links, rather than overburdening this page.
1. It appears that "lifted and separated" was the the ideal boob shape for most of the period I am interested in. This really cannot be accomplished with a fitted gown*, which gives you more of a mono-boob shape. Furthermore there is plenty of textual evidence for breast-shaping garments (and surgery!! WTF??) in the later middle ages which suggests to me that a breast-bag garment is more likely to have been employed than a super tight gown.
2. There is more than one way to bag a breast. The "long line bra" option is but one of four breast-bag garments that were found. It seems to have gotten the most press as it looks the most like a modern bra, but looking at visual sources as well as what is available on the find themselves suggests that many means of lifting and separating were possible. Just like we have lots of bra styles today, both for fashions sake and to accommodate different anatomy, they did the same thing back then (and why ever not? People in any given period of time face the same basic challenges and needs).
3. This one is my favorite - modern bras and bra making can teach us a whole heck of a lot about how to actually construct a plausible period breast bag. Again, the basic facts of anatomy have not changed. Breasts are what they are and there are some basic rules about constructing supportive and shaping garments that do not change regardless of time period or shape being sought. This is not to say a corset-is-a-corset or that a bra-is-a-bra, but in either case the garment is being supported at the waist line in the case of a corset or at the underbust line in the case of something bra-like.
Understanding the importance of the bra band and how to get the center point of a bra to tack (that is, sit flush against the rib cage) makes fitting a period style breast bag MUCH easier. Keeping in mind that my breasts are on the heavy side (another thing I would not have known without a foray into modern bra making), the bust band must 1 - exist and 2 - be snug/tight. This makes fitting the cups or bags much easier and proper support possible. It's also what does the lifting. The separating comes from how the cups/bags are arranged and fit on the body.
Once you figure out the band, you need to take a good hard look at your breasts and figure out where the root of the breast is (towards the center? under your arm? someplace in between?) and how much space you are working with at the center front. The first version of the Lengberg bra I made had way too much space between the cups, which prevented the band from tacking and allowed my breasts to creep out from under the cups. The current version has less space, but the band is still too loose so I am not taking but I am not falling out the bottom either. Figuring out where to make adjustments to get better fit is not as obvious (or at least wasn't to me) but there is a certain logic to bra fitting and it's easier to see after playing around with actual bra patterns. There are fitting instructions for these things after all, why not apply that accumulated knowledge?
4 - Tacking. As I mentioned above, the idea of tacking is that the center portion of a bra (where the two underwires come together on a modern bar) should lay flat against the sternum. This happens when the band is snug enough and the cups are sitting on the actual breast rather than starting too far to one side or the other of the breast root. A properly fit bra that tacks is a marvel and if you have never had this happen I highly encourage you to put down the computer, go find a specialty bra shop, and get fitted. With soft cup bras, this is not as likely to happen as there is nothing rigid for the band to pull against the body, but you should still not have inches of space between the band and your chest between the cups. Some things that seem to work to get this to happen on a breast bag type garment are 1 - making sure the band is snug 2 - getting the center height and width right and 3 - making sure you are working with the right shape garment for your particular breasts.
It's all about understanding your breast shape really. Just like with outer garments, you've got to work with the body you have. The first step in a well fit garment will always be understanding the shape of the body that will occupy it.
*assuming you have sufficient breast-mass to need help in this area