Re: top/bottom filters, I’m reading it as both the “Outer (Thinner)” acrylic piece and the actually mesh can be removed but the “Inner (Thicker)” acrylic cannot. Removing those two pieces might let the non-removable piece sit a few millimeters lower in the track, giving a tiny bit more clearance at the expense of filtration.
The outer thinner acrylic can be removed, obviously, in order to take out the filter to clean the dust. It is made thinner exactly for this purpose (ie you can bend it slightly by pulling out in the middle section to get the acrylic edges out of the frame inner track.
(This approach is just like so many air purifiers' filter system.)
The inner thicker acrylic, I think, can also be removed. It should better be described as 'not installed'. I mean, this acrylic cannot be bent like the outer thinner one. So, if you install it into the track (groove), it cannot be taken out easily (unless you disassemble the case). This means, you simply do not install it when assembling the case.
But, as the filter itself is clamped by an inner and an outer acrylic, not using one of the two will have the filter 'floating loose'. Some kind of glue or tape might be needed to fix the filter in place.
We will have to wait for confusis' review.