Julie,
I had assumed that the washroom door would open into your shop space, but it looks like it opens to the outside, so the suggestion I made earlier to place grilles in the door would not work. You need to return air back into the shop. You could open the window, but then if have heat in your shop in the winter or A/C in the summer, you will be drawing outside air back into the shop and losing energy. Based on your diagram above, I would cut a hole through the wall above the filter stack (near the motor level) to fit the grilles on the inside and outside of the wall. Line the inside of the hole with masonite to keep the cavity clean and put a low-cost filter in each grille. You will get the benefit of keeping the conditioned air in your workspace and improving air flow back to the shop.