The ornamental design for a collapsibleflotationdevice with back support, as shown and described.
FIG. 1 is a front perspective view of a collapsible flotation device with back support, according to an embodiment;
FIG. 2 is a top view of the collapsible flotation device with back support illustrated in FIG. 1;
FIG. 3 is a bottom view of the collapsible flotation device with back support illustrated in FIG. 1;
FIG. 4 is a left side view of the collapsible flotation device with back support illustrated in FIG. 1;
FIG. 5 is a right side view of the collapsible flotation device with back support illustrated in FIG. 1;
FIG. 6 is a rear view of the collapsible flotation device with back support illustrated in FIG. 1; and,
FIG. 7 is a front view of the collapsible flotation device with back support illustrated in FIG. 1.
The broken lines are directed to environment and are for illustrative purposes only and form no part of the claimed design. The thin solid lines illustrate surface contour and form no part of the claimed design.