The ornamental design for a convexbrush, as shown and described.
FIG. 1 is a perspective view;
FIG. 2 is an end view;
FIG. 3 is a top plan view;
FIG. 4 is a side elevational view;
The undisclosed end and side are the same in appearance as the respective
end and side shown in FIGS. 2 and 4 and the undisclosed bottom is flat and
unornamented.