The ornamental design for an armcastcover, as shown and described.
FIG. 1 is a top perspective view of an arm cast cover in accordance with the present invention disposed on a human arm;
FIG. 2 is a top view of the arm cast cover;
FIG. 3 is a bottom view thereof;
FIG. 4 is a right-side view thereof;
FIG. 5 is a left-side view thereof;
FIG. 6 is a rear view thereof; and,
FIG. 7 is a front view thereof, inverted relative to the view presented in FIG. 6.
The broken lines in FIG. 1 present unclaimed subject matter for the purpose of illustrating the environment in which the article embodying the design is used and form no part of the claimed design.