Jack, lots of good (and some not so good) advice here on sled runners. I would like to make a case for using a single runner. Consider that the center to center spacing of two runners will not be EXACTLY the same as the spacing of the saw slots. The sled spacing will, of course, change with changes in humidity. Say the runner spacing is greater than the miter slot spacing. The sled will be guided by the outside edges of the two runners. The inside edges of the runners never make contact with the slots and thus these two edges have ZERO affect in guiding the motion of the sled. Similarly, if the runner spacing is less than the slot spacing the inside edges of the runners alone guide the sled. I have heard comments that a double runner sled is more accurate because there are twice the number of guiding surfaces. Is NOT true! A single runner also has one left and one right guiding edge as well. As a matter of fact, a single runner sled can be made more accurate for the amount of free play does not have to be large enough to accommodate the changes in the runner spacing with humidity change. An advantage of a single runner is you can use the runner in one slot for zero-clearance applications and that same runner in the other slot for dado and bevel use. And yes, that sled is easier to construct.