The mallets above are based on the designs of a specific professional line of synthetic-core marimba mallets; the very soft (no ring), medium soft (green ring), and medium (blue ring) mallets use the same cores already present in the original sticks, but the soft (orange ring) mallets are an intermediate core hardness unavailable in the storebought line. They're wrapped in four-ply Wool-Ease yarn (from Lion Brand Yarn and available at most craft stores). I like the four-ply yarn marginally less than the three-ply sportweight Wool-Ease, but Lion stopped manufacturing the three-ply stuff several years ago, while the four-ply stuff is easily available and sounds very good. (I have one or two skeins of the three-ply yarn secreted in various places, waiting for the right project to come along—once it's gone, it's gone.) The rounder mallet heads result from an intermediate yarn layer that is wound at a steeper angle. The unwrapped cores are pictured below; each successively harder mallet has a total thickness of latex that is 1/16" less than its neighbor to the left, and the weight of each mallet decreases a bit for each step harder you go.
• Extra Soft: 1" poly ball core, 3/16" latex layer, 1/8" latex layer (5/16" total)
• Soft: 1 1/8" poly ball core, 1/8" latex layer, 1/8" latex layer (1/4" total)
• Medium Soft: 1 1/8" poly ball core, 1/16" latex layer, 1/8" latex layer (3/16" total)
• Medium: 1 1/8" poly ball core, 1/8" latex layer
The smaller 1" core on the softest mallet allows for a large amount of latex overlay that still results in a finished mallet that isn't comically large. One could conceivably make a medium hard mallet with a 1/16" latex layer—indeed, the storebought series goes on to do just that—but that little rubber over a hard plastic core results in a mallet that it very light and very bright—more so than is useful for me.
It used to be that you could buy poly-ball-and-dowel mallets from Steve Weiss Music for $5 a pair; those were the days. These mounted cores were made for me by Smith Mallets—if you send them an email, the folks at Smith is happy to put their poly cores on concert-length birch for a very reasonable price.