I have always used surcingles for the same reasons. I have just always made my own and like them a lot better than the ones you can buy.
I take 2 western cinchas and attach an off billet to the off side to buckle them together and use a regular leather or nylon tie strap on the left side to cinch them together just like putting a saddle on. I use a wide roping cincha over the horse's withers and a narrow mohair girth under its belly. The cinch rings work very well to attach driving rings to.
I am using one right now to make sure of my 3year old's steering/brakes before I get on him later this spring. Plus he's gotten used to having the lines rubbing his legs/sides/over his butt and flapping on him when I roll them up and lay them over his back.
I wish that mine had a couple of rings lower on his side, but it works.
I used one to train my dressage mare... the whole long-lining thing didn't work out so well (I had more trouble with it than she did) but I did lounge her with a surcingle and side reins for awhile. Also used a surcingle and side reins to teach her how to cross her hind legs and turn of the forehand.
Expect some resistance at first, but they do work pretty well.