I also own an lPL, a 4550 XLG. My wife made me one for my birthday a few years ago. She made it with standard weight matte board and used our matte cutter to bevel the edges. It works like a champ.
She hates the look, but I love it. Guess who does the printing.
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.
The standard neg carrier on my Leica 1C is a doosie, thin but a little rough as well. The carrier on the V35 is more precise but a little too wide for my taste.