The ornamental design for a modular basepanel, as shown and described.
1. Modular base panel
1.1: Perspective
1.2: Enlarged
1.3: Top
1.4: Front
1.5: Back
1.6: Right
1.7: Left
The dot-dashed broken lines in reproductions 1.1and 1.2and the associated number in reproduction 1.1are only to represent enlarged portions of the claimed design and form no part of the claimed design.