How smoothly does it need to turn, and how much weight does it need to bear? How much difference is there in the sizes of the two cylinders? You could make a bearing: pack the space between the inner & outer cylinders with something round and uniform. The spheres (rods may work too) should be the same size and fill the available space, preferably so only 1 layer can fit between the inner and outer cylinders. (Take apart a bike wheel axle to see what I mean.) If you really need a railing rather than a bearing, you might be able to cut a flat "halo" from a sheet of plastic and screw or glue it inside the outer cylinder, then use small pulleys to roll along the inside edge of the halo, with the pulleys screwed or glued to the inner cylinder. If you're working with sheet metal, be careful of sharp edges.

Apart from the observation that such things exist, I'm not sure how to answer your question. What material is the cylinder to be made out of? How tall is it (and how massive)? How rapidly is it intended to spin? The material strength of the cylinder and of the railings, as well as the type of fixture you want, depend upon these. For example, if your system is expected to be heavy but you don't mind losing angular momentum to friction, then it might be possible to build it so that the cylinder is held in place by gravity, like the gun turret of a tank or battleship. This obviously isn't going to work if the cylinder is light enough or will be rotating at such speeds that it might fly up out of the pit you have prepared for it.