We claim the ornamental design for the add-on enclosure for outdoor boiler, as shown and described.
FIG. 1 is an isometric view of the add-on enclosure for outdoor boiler, showing our new design;
FIG. 2 is a front-elevational view thereof;
FIG. 3 is a right side-elevational view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a left side-elevational view thereof;
FIG. 6 is a rear-elevational view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines in the figures are for illustrative purposes only and form no part of the claimed design. Instead, the claimed design extends to the broken lines yet does not include the broken lines.