from matplotlib.path import Path

def custom_box_style(x0, y0, width, height, mutation_size, mutation_aspect=1):
    """
    Given the location and size of the box, return the path of
    the box around it.
    
     - *x0*, *y0*, *width*, *height* : location and size of the box
     - *mutation_size* : a reference scale for the mutation.
     - *aspect_ratio* : aspect-ration for the mutation.
    """

    # note that we are ignoring mutation_aspect. This is okay in general.

    # padding
    mypad = 0.3
    pad = mutation_size * mypad

    # width and height with padding added.
    width, height = width + 2.*pad, \
                    height + 2.*pad,

    # boundary of the padded box
    x0, y0 = x0-pad, y0-pad,
    x1, y1 = x0+width, y0 + height

    cp = [(x0, y0),
          (x1, y0), (x1, y1), (x0, y1),
          (x0-pad, (y0+y1)/2.), (x0, y0),
          (x0, y0)]

    com = [Path.MOVETO,
           Path.LINETO, Path.LINETO, Path.LINETO,
           Path.LINETO, Path.LINETO,
           Path.CLOSEPOLY]

    path = Path(cp, com)

    return path


import matplotlib.pyplot as plt

plt.figure(1, figsize=(3,3))
ax = plt.subplot(111)
ax.text(0.5, 0.5, "Test", size=30, va="center", ha="center",
        bbox=dict(boxstyle=custom_box_style, alpha=0.2))