This poster describes the design of a unit called Tx Attenuating Photoreceiver (TAP), dedicated to the measuring and dumping of a 2 W beam with 300 mm diameter. This unit is part of an optical demonstrator which will be created for stray light testing in the LISA mission. The geometry of the instrument is studied to reduce the size of the beam, focus it onto a photodiode, dump its power and minimize scattering and reflections back to the device under test. The setup will be composed by an off-axis parabolic absorber with anti-reflection coating which will absorb most of the beam, reflecting only 0.2 % towards a pinhole and a photodiode.