The ornamental design for a interlocking elongate flotationdevice, as shown and described.
FIG. 1 is a front elevation view of a interlocking elongate flotation device, the device being cylindrical and, thus, having identical front, rear, top, and bottom elevation views;
FIG. 2 is a left end elevation view thereof;
FIG. 3 is a right end elevation view thereof;
FIG. 4 is a perspective view thereof from a right end; and,
FIG. 5 is a perspective view thereof from a left end.
The portions of the interlocking elongate flotation device shown in broken lines form no part of the claimed design.