I make mine out of cardboard (and I made one out of a DVD-cover), I simply measured up the correct dimensions (my carriers are normally metal insets into a unineg-holder) and cut them out using a metal ruler and/or freehand cutting (to get some unevenness).

The one made with the dvd-cover, can be adjusted using regular black electrical tape,

Took a bit of trial and error, but I now have a couple of carriers that give me a border effect and I still have my regular/factory carriers in good condition.