The plastic top is not a separate part.
There are a lot of different part numbers, because these springs are used on a lot of models. Google "Mustang trunk springs" and you'll get
something like this or
this. Or go to a junk yard and get them for a few bucks. Or maybe free.
Sinovac did the legwork at Lowe's to find this piece that fits into the spring if you don't want to replace the entire part.
View attachment 54383
I'm surprised he used them, because they are made in Taiwan. Maybe that's okay, because it's not mainland China. Minus a point at the Concours judging.