I've made several disposable, job specific router sleds over the years, then dismantled them after the job was done. All of them have used wood runners that straddle the workpiece. As I contemplated building a new sled to flatten some slabs it occurred to me that there is no need for the rails if you have a flat bench that's wider than your workpiece. So I built this simple sled that rides directly on the edges of the workbench.
I sized the runners at 2-3/8" to clear my 2-1/4" thick slabs, and added side guides to keep the sled from slipping off the benchtop. It worked just as expected after I shimmed the workpiece steady on my workbench.
But I sure could use a dust/chip collection system to go with it, it makes a heck of a mess, so if anyone has devised one please pass it along.
John