On Wooden runners, you are asking for movement and trouble ahead if you don't use QS wood with the grain presented with it running up and down as it sits in the slot. That way the wood movement will possibly make your sled taller or shorter, but won't make the wood bind in the slot due to seasonal changes. I'd suggest never to use flat sawn wood, as it will definitely bind up if it moves, or at least gets overly snug.
Pics below to show what I mean here, in case the words didn't do it.
Any of the above runners could work. The plywood slice is "Dimensionally stable" so it shouldn't move. I don't suggest it's use on a runner though as my experience shows a much higher likelihood of delamination on very small slices of plywood, so while it may seem stable, it, with the addition of screws will likely delaminate. Both of the other pieces are heavily rift sawn pieces, and likely won't have much movement side to side. So again the taller, or shorter sled won't effect operation, except for needing more blade height. True QS would be better IMHO though.
Probably will work fine
IF you have a slotted miter slot, that is used as a safety feature to prevent tipping of the gauge, or a sled. I wouldn't ever suggest trying to fill the lower slotted portion with wood, even QS. Instead make the QS runner just to fit the upper portion of the slot, and if you wanted the safety feature of using the lower slot, then screw a washer to the bottom of the runner, so it engages the wider slot. You will have an easier time of it that way.
My thoughts on this and 75 cents may get you a weak cup of coffee at a diner.
