Tim, what I have done for years is this: Buy some B/W sheet film, take one outside into the sun, develop it, it will be black. I then get a sheet of graph paper, measuring very exactly the size of the mask I need onto it, then I tape the film sheet to it and cut (with very very very sharp blade) through both film & paper from the paper side. This is probably the easiest & cheapest method. No problems with the neg carriers not closing! Once they're made, look after them & they'll last for years.
-David.