Its design is incredibly simple: a single peice of neoprene with a single seam at the back stitched together using a zigzag stitch.
As is evident from the first picture, it's starting to show its age a bit. Some of the foam material has started to break down so it doesn't feel quite as thick as it used to. I'd have bought a replacement years ago if I could find one, but have never seen anyone selling anything like it. Last year Heather and I started doing more winter rides and it became apparent that her fleece headbands were often not enough. It occurred to me that my headband should be quite easy to replicate. Neoprene fabric can be bought in small quantities online from various sources. I bought a 1 foot by 4 foot piece of it from an eBay seller for about $15. I estimated the thickness of the fabric used in the original to be about 2 mm thick, so I ordered the material in that thickness, but it appears to be a slightly thicker than the original. It's possible the original was actually 1.5 mm (I think the fabric is also available in that thickness), or that it's just thinner due to deterioration over time. In any case, 2 mm seems like about the right thickness for me, but others may prefer something in a different thickness.
I have a relatively large head, and Heather's is relatively small, so based on the original headband, I created two separate patterns, to fit large and small head sizes. In case this is useful to anyone, I created this PDF document with patterns for both sizes.
Note that the patterns reach relatively close to the edges of letter-sized paper, so check the print settings when printing it out. Some viewers by default will try to shrink it to add extra margin at the edges. In Chrome PDF viewer, uncheck the "fit to page" option. To make sure the printout is the right size, I've added measurements to the diagram so the size of the printed pattern can be checked before using it to cut fabric.