A problem like that would drive to answer no matter what it took. I have been known to use 1/2" angle iron in cases where I wanted to stiffen a panel. Top and bottom covers especially. I would look for a way to do this in a case such as this as well. Sometimes a thick brass bar that fits is all it takes. Drill 6 holes for #6 bolts through the panel that needs the support and thread the brass bar. get mean.
I just took the whole thing apart so I could flip the base plate and now I see how the heatsinks are slightly off which is skewing everything else. Before I did that the washers helped fix the rear panel. It wasn't flush, but it was a lot closer than without them.