This figure shows the stuctural design of the propulsion module. The main driver is the very small available height. For thermal reasons the solar array panel cannot be used for unit accomodation. Therefore all the propulsion module units have to be accomodated on the module rim.
Instead of the triangular shear walls, a cylider is used in order to also provide a stiffening of the panel at the outer rim. This is still not sufficient, additional webs have to be used locally at unit mounting positions. The top plate is needed for the solar cell fixation.
From LISA System and Technology Study Report (2000) (Fig 9.9)
ESA-SCI(2000)11