def get_inner_volume(wp):
    return\
      wp\
      .rect(50,50)\
      .extrude(50)\
      .edges()\
      .fillet(10)      
    

def split_bottom(
      wp,
      cut_zoffset):
    return\
        wp\
        .workplane(cut_zoffset)\
        .split(keepBottom=True)
        
def get_case_bottom(wp):
    return\
      split_bottom(
        get_inner_volume(wp)
        .union(get_inner_volume(wp.move(25,25)))\
        .shell(2),
        0)
          
def add_inner_lip(wp, width):
    return\
    wp\
    .faces(">Z")\
    .wires()\
    .item(0)\
    .toPending()\
    .workplane()\
    .offset2D(-width)\
    .extrude(1)\
    .faces(">Z[-2]")\
    .wires()\
    .item(2)\
    .toPending()\
    .workplane()\
    .cutBlind(1)

result = \
    add_inner_lip(
        get_case_bottom(
            cq.Workplane("XY")),
        0.9)