To add to @dlang’s response, the typical sled is 18" in diameter. Plywood works pretty well for the structural part of the sled, but it can create a lot of friction. I upgraded my most recent sled to have a HDPE plastic skid pad and it has helped immensely. With a good 1/4" round-over, the sled will even slip right over bumps where it would have caught before. It will even slide over my fence better, so I can now use the bottom 10" of my bed. 
You should be able to retrofit a round sled with the ring kit. See this discussion for more details and a template for mounting the ring kit. Personally, I would just make a new sled with the holes predrilled from your dialed in Maslow. The plywood is cheap enough and it makes assembly just soo much easier. 