Report:
I bought the A12 light trap kit on Ebay for $13. It arrived today and I installed it. The instructions were good, clear, and there were good photos. The materials consist of a sliver of die cut mylar with a crease, and a sliver of gray foam to go in the crease. About 5-cents worth of materials, but I am sure the die to cut the mylar was a few hundred dollars. So, price was fair enough. Indeed the old trap was fully rotted away. The NINE tiny screws had me a little worried and sure enough one was frozen. Always happens, doesn't it? I carefully worked the stuck screw with some added leverage and thankfully it loosened.
I went right out and shot a roll and sent it off. Hopefully, this will fix the light leak. We'll see. The dark slide did feel tighter going in, so....
In the future, now that I see what the deal is, I will make my own, and save the $13. The nice die-cut mylar isn't going to wear out, just the sliver of gray foam will rot away.