The floor of the accumulator is treated like any other wall.
Openings are legal per F.10.4.
Please note we take the T.7.1.2 ban on powered ground effect very seriously.
As long as your team does not attempt to vary or control downforce or drag performance with the fans, we will not consider this a powered ground effect.
Reasonably sized cooling fans steadily flowing air through the blockages of electronics and cells should have minimal vehicle aerodynamic effects.
Don't push it, and you should be fine.