I dug mine out, thought had contact details but don't. Were done by a guy on ebay probably 10 years ago. Clear film over white backing with windows. I think many done copy the codes and specs off the 1 or 2 done on groups previously. Have seen a few original vs new comparison pics over the years where they didn't even supply the details they had, probably just the link to some other car and insert VIN. Not that the details matter, but if you have them...
I see Firebird Central sell blank transfers with windows cut out and separate clear overlay, another way to do it. But still have to type your details in and risk messing it up. But the idea works if doing a custom transfer too.
Also, don't drop them in the dirt while applying. I did that years ago, clear transfer doesn't look great with pebbles under it. No really, it doesn't.