Somewhere somehow I bet you picked up or where given a union suit. May have been a real or imagined need, or a present. Damn me if they are not uncomfortable though (wedgie) and impractical (sweaty cold cotton/poopshoot). A thermal undershirt though -that might get some use. Dis-join the suit below the rear button (you know, for a feature...) in an arc. Still long enough to tuck in, and then some.
the backflap button and reinforcing tab.
Interior contrsuction of that area -double layer flap.
Stitch it down.
Final back. 'Shoot button left at bottom.
Just throw a red flannel on top and you are ready for the spring drive.
Or could sew the placket up the front and make it more like this.