I have started using "pinky
slides" which barely cover 6 strings, but are lighter and more manageable with the pinky. Dunlop has a straight chrome one, JetSlide's brass one is also "straight" on the outside, but slightly tapered on the inside, so fits comforably.
As far as a "ring-like" slide, you might try a "knuckle slide", a la Miss. Fred McDowell. It covers about one knuckle (maybe 3 strings?) and doesn't really need to be concave since it is only going to be touching 1-3 strings at a time. I think that Dunlop makes some sort of knuckle slide, and probably in various inner diameters, so you could find one for your ring finger.