I'd expect the shower manufacturer to provides some guidelines.
Two attachment points or 8 attachment points ?
What is the Ø of the mounting holes on the shower?
Are there any struts etc behind the aluminum "plate" that the shower could be attached to? Or that a painted plywood panel could be attached to and still maintain the desired mounting location for the shower?
Should corrosion be a concern?
Define "aluminum plate".
1/2" thick ? not much fun with self tapping screws, but if tapped manually 2D thread engagement for 1/4" or M6 fasteners.
Mind the 220 volt cables mounted to the hidden side of the plate.
.018" thick ? Easy for sheet metal screws, but very limited holding power.
Maybe moly bolts or something similar.
I guess my question was the general nature of metal screws. Do I absolutely have to get a nut under there or can the threads hold the equipment in place. It will probably be a quarter inch. Robust enough to walk on
You can locally reinforce the Al plate by welding additional Al plate on top to have enough thickness for cap screws to latch on, provided it is possible under your condition.
Engineers, think what we have done to the environment !