My suggestion as mentioned on numerous Anovos build threads, if you have plenty of room between your arm and the armor only trim so that your left with roughly 8mm flat surface on each side for the cover strip, starting with the forward facing (or upward) surface. If you have to take a little off at a time, it's much easier to remove material then it is to add it.
Even if it's loose when assembled just add fabric foam to the interior when finished. The forearms are proportional to the biceps and bells, same with the legs. If you trim too much off one area the armor will look odd when you're done.