Those you will find here, offered by other members, will most likely be 3d printed. I know of no others. And you certainly could find those "that really don't work" but it depends on several factors. the primary factor is the part should properly fit the coil. second factor is design, and how it is "built" to do it's job.
I've found one scenario where you will get an obvious "working" result. that is the coil ear tabs are broken off. what works here is that the part will fill in for those tabs, and the coil is attached back and swinging fine. works best if even broken tabs are kept in place (depending on stiffener design). they are mated to the coil using zip-ties but could also be effectivley glued into place.
the preventative measure of these stiffeners is tough to gauge. I'm not willing to pressure my coil to confirm the working nature of the stiffeners, but I've accidentally given the coils a good push once or twice and marveled that they didn't break (I do have stiffeners on my coils).
another big difference I've seen, at least with the equinox coils, of those stiffeners I've seen here and on fleabay, is the ability to cover more than one coil size or type. this is a fallacy. each coil is unique. for example, the 3 found with an equinox, 6, 11, 15 inch coils each require their own stiffener though sometimes they can be close fits.
and for a suggestion to a good equinox choice (and I think other models too) here is a thread where you can find a link to the stiffener one member offers:
http://metaldetectingforum.com/showthread.php?t=291701
