After seeing the pictures of 1912 bras or Bandeaus I thought, I can do that.
Once the slip pattern has been properly fitted for you you can use that, but I used a princess bodice pattern that I know fits me. I'm a front fastener person due to arthritis in my shoulders so adapted the pattern for that.
Cut it out from a piece of pink slipper satin I've had forever. A fitting tip--sew the back and side seams, the front seams up to the breast points and make those fit. Then put on the closure, try on the bandeau inside out and pin the upper breast seams. Because of my particular shape the upper part of the cups is always too big so those seams were about twice as wide as normal.
It is way too high at the back neckline for a gown, but should be fine under a blouse.
And it is comfortable! No pinchy elastic!