Regarding the question about why I would need washers:
When you put the MAF screen in, you're basically wedging it between the PVC pipe and the MAF pipe, which creates a small 1mm gap between the pipes. considering that the pipe is circular and the base of those pipes is a square, you're bolting them together at the corner. With the maf screen wedged in there, when you try to tighten the bolts all the way, the base flexes at the corner, trying to put the pieces of plastic flush, when the rest of the base is 1mm apart. This warps the plastic slightly, which can break the seal on the maf. So by placing small washers at each bolt hole of the same thickness as the maf screen, you're allowing everything to be bolted flush. hope that makes sense.
a good analogy would be if you were trying to bolt two pieces of metal flush together. if you stuck a pencil in between the two pieces of metal, then tried to bolt them together, the metal would flex. but by placing spacers (washers) at the bolt spot, everything can be flush and parallel again.