The ornamental design for an inflatable back cushion, as shown and described.
FIG. 1 is a front left perspective view of an inflatable back cushion in a folded-up configuration (and at least partially inflated), and in accordance with one embodiment of the present invention;
FIG. 2 is a front elevational view of the inflatable back cushion of FIG. 1;
FIG. 3 is a right side elevational view of the inflatable back cushion of FIG. 1;
FIG. 4 is a left side elevational view of the inflatable back cushion of FIG. 1;
FIG. 5 is a rear elevational view of the inflatable back cushion of FIG. 1;
FIG. 6 is a top plan view of the inflatable back cushion of FIG. 1;
FIG. 7 is a bottom plan view of the inflatable back cushion of FIG. 1;
FIG. 8 is a rear elevational view of the inflatable back cushion of FIG. 1, but shown in an unfolded or extended configuration;
FIG. 9 is a front elevational view of the inflatable back cushion in the same configuration of FIG. 8; and,
FIG. 10 is a front perspective view of the inflatable back cushion of FIG. 1, but shown in another configuration from a modified angle for further clarity.
The broken lines in the figures depict the portions of the inflatable back cushion that form no part of the claimed design.