I guess it depends on the actual seat used, if they have any inbuilt headrest mounting location for a bracket or to secure to, after that it only needs to be a loop of sorts, off any number of similar seat setups. What seats are in yours?
I ran across this video awhile back. He has the same aftermarket seats that I have and utilizes a seat belt guide from a 2005-2010 Chevy Cobalt or Pontiac G5. The headrest posts hold the guide in place. https://www.youtube.com/watch?v=Zz7KqPAoVRcI tried locating these guides at one point but couldn't find them anywhere. Although now that I've got the car on the road and am using the 3-point seat belts, I don't really find a need for the guide.