My diffuser was way to brittle, my patience way to short, and my tool-set way too small to perform most of the suggestions already posted.
I was able to heat a blade (box cutter) with a small torch (a big torch lighter); making quick work of cutting along a line.
I metal strait edge below the diffuser helps keep the line strait.
You'll have to develop your cut-and-heat technique as you go.
I went between heating the mid-point of the blade, and plunging the blade into the line; cutting an inch with each plunge (every 3 or 4 seconds).
Edges weren't perfect, yet good enough for my purposes.