A modular mold assembly (10; 210) comprising: a base (14; 214) having a mold surface (30, 30A, 30B, 30C) that partially defines a mold cavity (20, 20A, 20B, 20C) for thermoforming a polymeric bladder with a fluid-filled chamber; wherein the base (14; 214) includes a side wall defining a first opening (22A, 22B, 22C) that opens into the mold cavity (20, 20A, 20B, 20C); and a first removable mold tool (18, 18A, 18C, 18E) that has an inner wall (31, 31A, 31C, 31E) with a mold surface (32, 32A, 32C, 32E) having a topography different than a topography of the mold surface (30, 30A, 30B, 30C) of the base (14; 214); wherein the first removable mold tool (18, 18A, 18C, 18E) fits in the first opening (22A, 22B, 22C) in the side wall of the base (14; 214) with less than a predetermined clearance and with the mold surface (32, 32A, 32C, 32E) of the first removable mold tool (18, 18A, 18C, 18E) adjacent to the mold surface (30, 30A, 30B, 30C) of the base (14; 214) and further defining the mold cavity (20, 20A, 20B, 20C); wherein the first removable mold tool (18, 18A, 18C, 18E) includes an outer wall opposite the inner wall (31, 31A, 31C, 31E), a front wall, a rear wall opposite the front wall, a top wall, and a bottom wall opposite the top wall; wherein each of the front wall, the rear wall, the top wall, and the bottom wall extends from the outer wall to the mold surface (32, 32A, 32C, 32E); and wherein the rear wall is wider than the front wall and the bottom wall is wider than the top wall, wherein the first removable mold tool (18, 18A, 18C, 18E) defines channels extending through the first removable mold tool (18, 18A, 18C, 18E) to the mold surface (32, 32A, 32C, 32E) of the first removable mold tool (18, 18A, 18C, 18E). a base (14; 214) having a mold surface (30, 30A, 30B, 30C) that partially defines a mold cavity (20, 20A, 20B, 20C) for thermoforming a polymeric bladder with a fluid-filled chamber; wherein the base (14; 214) includes a side wall defining a first opening (22A, 22B, 22C) that opens into the mold cavity (20, 20A, 20B, 20C); and a first removable mold tool (18, 18A, 18C, 18E) that has an inner wall (31, 31A, 31C, 31E) with a mold surface (32, 32A, 32C, 32E) having a topography different than a topography of the mold surface (30, 30A, 30B, 30C) of the base (14; 214); wherein the first removable mold tool (18, 18A, 18C, 18E) fits in the first opening (22A, 22B, 22C) in the side wall of the base (14; 214) with less than a predetermined clearance and with the mold surface (32, 32A, 32C, 32E) of the first removable mold tool (18, 18A, 18C, 18E) adjacent to the mold surface (30, 30A, 30B, 30C) of the base (14; 214) and further defining the mold cavity (20, 20A, 20B, 20C); wherein the first removable mold tool (18, 18A, 18C, 18E) includes an outer wall opposite the inner wall (31, 31A, 31C, 31E), a front wall, a rear wall opposite the front wall, a top wall, and a bottom wall opposite the top wall; wherein each of the front wall, the rear wall, the top wall, and the bottom wall extends from the outer wall to the mold surface (32, 32A, 32C, 32E); and wherein the rear wall is wider than the front wall and the bottom wall is wider than the top wall, wherein the first removable mold tool (18, 18A, 18C, 18E) defines channels extending through the first removable mold tool (18, 18A, 18C, 18E) to the mold surface (32, 32A, 32C, 32E) of the first removable mold tool (18, 18A, 18C, 18E).