Take a look at this picture of a PCI-Express x1 slot (at the end of a flexible extender) that was cut out to allow down-plugging a PCIe x16 card into it.
I noticed that when people use a Dremel to cut the plastic, they tend to intuitively place the cutting disk along the longitudital axis, often damaging the pins in the slot because of the thickness of the cutting disk.
Instead it is easier to place the cutting disk along the lateral axis. This removes more plastic material, but does not necessitate a really precise cut with a thin disk and steady hand :-)