The baffle supports must be designed to handle the torque input by the mixer.  Mixer torque should be calculated using the motor power and dividing by pi times the mixer shaft speed, with the proper conversion factor for units.  The torque formula for power in horsepower, shaft speed in revolutions per minute, and torque in inch-pounds is torque [in-lbs] = 63025 * motor horsepower [hp] / output shaft speed [rpm].  Note that the correct speed to use is the mixer shaft speed, not the motor speed.  A speed reducer is a constant power device and a torque increaser.

A variety of methods can be used to get from the input torque to the baffle loads.  The simplest method is to simply distribute the torque input to a force load on each baffle, dividing by the number of baffles and the distance from the tank center to the tips of the baffles.  Then the baffle thickness, supports shape, number of supports, etc. must be designed with adequate strength.   Using maximum design limits of 6,000 psi for shear stresses and 10,000  psi for tensile stress levels usually works well for steels, carbon or  stainless.  These strength levels take into account that the failures will occur because of fatigue, not yield strength.

The dimensions given are not sufficient to provide more detailed calculation.