I started by masking off the spot for the black patch. I used the edge seams as a guide, and the elastic itself settled very well centered to begin with, making the width easy to square up.
If you compare this to the first picture, you'll notice that I made the patches a little taller by taking off the second half-inch of tape. The patches look like they are actually quite big on screen, but I couldn't source any info on actual measurements. So I'm eyeballing it. I figure I still want to err on the side of small. I can always add more.
If you look at the left picture here, you can see that on Venkman's pads, the black almost looks like it's full height. But, as is common, it varies and depends on the shots you're looking at.
Just used some plain old legal pad paper I had handy to mask off the body of the pads. Outside they went for spraying...
Here they are after spraying. My magic sauce is Krylon Fusion. I've always had good luck with it on... pretty much everything. This stuff is miracle paint, for the most part. Left it out in the sun for about ten minutes to dry. The elastic will be a little stiff afterwards, but it's fine and will loosen right back up.
And here's the outcome! One looks taller than the other, but that's just an optical illusion of the angle the photo was taken at. The Fusion coated magnificently and evenly. The color looks right, and it makes them pop the right way. I do think I may go back and add a 1/4" or so to the size as I go back and forth.
The thing I love about this approach is A) I don't have a sewing machine, and suck at sewing by hand, and B) no extra bulk from an additional elastic patch on the inside of the elbow.