I have a custom surcingle with turrets (fixed, raised rings on top); it’s designed so that it doesn’t contact the spine - not even the padding (similar to saddle panels). Doesn’t have a bunch of extra rings, just a couple of small d-rings on the leading edge, near the billets.
I use a regular string girth, with a lightweight quilted saddle pad, and the surcingle doesn’t slip (even on my round horses). I’ve had my surcingle about 30 years, after using one from the same maker at my trainer’s, and it’s worn like iron even though it’s received quite a bit of use. Back in the day, if I participated in a clinic where others saw my set-up, the maker would get a flurry of orders (she told me so).
I ordered my most-often-used lines at the same time – they’re the flat cotton web type with the rolled black nylon sections that run through the turrets and bit rings, and they’ve held up very well. After lessons with Victor Silva years ago, I also made up a shorter set, from cotton rope, as that is what he preferred with my surcingle when teaching me at the time, but they rarely leave the tack room. Alfredo Hernandez used my usual long set when working with me and my horse, as have the others from whom I’ve learned.
I’ve always tried to take advantage of clinicians’ long-lining knowledge, when possible, as I really enjoy it and love to see my horses as they work. It’s so helpful, too.
It’s extremely important to always have safety at the forefront of one’s mind when long-lining, to avoid entanglement. I had one trainer who liked to throw the bight (excess line length) over the shoulder to keep it out of the way, but I refused to do so, as I don’t want a line anywhere near my neck.