The human voltaren is supposed to be not as well absorbed by horses as the horse formulation. There is a study floating around (that has been cited on these forums) that demonstrates absoprtion of various formulations in horses. I’ve used the human version and have seen results, so it does something but you should assume it’s less efficacious (and might then need to use more and/or administer more frequently). But if your horse is truly suffering from KS pain, I don’t think there is any topical out there that is going to help enough.
Not exactly what you asked, but honestly, if your horse is sore that quickly after (presumably steroid) injections, you might consider either 1) other more invasive options such as surgery (what have you tried so far?), or 2) other sources of pain that then radiate and show up in the back. Have you radiographed the hind feet? Negative angles in the hind feet can absolutely show up as back pain (ask me how I know!), as can neck pain. I would join the facebook group on KS in horses if you haven’t already. Lots of knowledge in there. KS is such an easy diagnosis as it’s easy to tell when a horse is back sore, and easy to “confirm” with radiographs, even though the back pain might be secondary to something else.
I have a horse diagnosed with (admittedly mild) KS that seems to be resolving itself with a “physical therapy” approach combined with fixing NPA in the hind feet. Like you, I injected the back on my horse thinking that was the issue, but a few weeks later the horse was uncomfortable again. I switched to a physical therapy approach (teaching proper posture, core strengthening, etc) while I waited for the NPA to resolve, and every time I fix the feet (changed farriers, put wedges on, took wedges down, put them back on), the horse changed accordingly - got better, better even yet, got worse, and then better again - so I am 95% sure most of my horse’s pain was foot related, and the KS was just a red herring (TBD if this is true over the long term, but right now am I thrilled how much better she feels after just fixing the feet).