Just a note on the Prado Packers. They ( the OME95PF5's) are rumoured and seemingly advertised to be 5mm high but measuring them with my micrometer showed they were 4mm high.
After trimming down, the internal diameter of the packer should end up about 56mm to fit over the shock collar. The ring inself will be about 3.48mm thick at that point.
I found that taping them tightly together (around their outside circumference as recommended by cmohr) worked well and allowed them to be trimmed down together thus saving some time (just trim off any tape that laps over the outside edge). Even with a Dremel tool it does take some time to remove enough material to get it to fit over the shock. I was going through all of the grinder wheel attachments that I had trying to trim it down. I found that a 40mm diametre drill chuck flap wheel in a very course grit (like 60) makes good progress (better than the Dremel) but it does make the ring very hot.
If you can, put your electric drill into a vise or clamp it to something solid - with the flap wheel installed in the drill you will be able to just concentrate on holding the ring assembly (with leather gloves is recommended due to the heat the ring absorbs).
Because the grinding and sanding removes the coating on the inside, I painted mine so that they will not start to rust.
They are ready to go in as soon as the rain stops.