I claim the ornamental design for a flat bag with bottle holder, as shown and described.
FIG. 1 is a three-dimensional view of the flat bag with bottle holder.
FIG. 2 is a front view of the flat bag with bottle holder.
FIG. 3 is a rear view of the flat bag with bottle holder.
FIG. 4 is a left side view of the flat bag with bottle holder.
FIG. 5 is a right side view of the flat bag with bottle holder.
FIG. 6 is a top view of the flat bag with bottle holder.
FIG. 7 is a bottom view of the flat bag with bottle holder.
FIG. 8 is a three-dimensional view of the flat bag with bottle holder with the bottle pocket open; and,
FIG. 9 is a cut-away view of the flat bag with bottle holder with the bottle pocket open.
The dashed broken lines show portions of the flat bag with bottle holder that form no part of the claimed design. The dotted broken lines show stitching that is included in the claimed design. The solid waved lines shown on the end of the straps in FIGS. 1-9 depict symbolic break lines and form no part of the claimed design.