The ornamental design for a portable bat rack with under flange for insertion into a common paint or spackle can, as shown and described.
FIG. 1 is a top plan view of a portable bat rack with under flange for insertion into a common paint or spackle can showing my new design;
FIG. 2 is a front elevational view thereof, the rear and sides adjacent being indentical in appearance;
FIG. 3 is a bottom plan view thereof; and,
FIG. 4 is a top perspective thereof, with the broken-line disclosure of baseball bats and a spackle can being for illustrative purposes only and forming no part of the claimed design.