The ‘wing’ frame gives the motors a lot of leverage (it puts the motors several inches out from the arms, and the arms 18 inches above the plywood, and it’s the plywood and all the screws that need to resist flexing)
You want to give the chains/motors as small a lever arm as possible against the pull of the chains towards each other.
Ideally, you do this by mounting the bracket so that it’s flush with the front of a solid beam (wood or unistrut) that connects the motors (unistrut is not quite as common as 2x, but it’s not far behind, I think the design should support both)
- Chain positioning
The ‘wing’ frame forces you to compromise between the chain being parallel to the workpiece, and it being at the balance point of the sled. The stock motor mounts put it at a reasonable place for working on 1/4"-1" material, but beyond that (including thick waste boards), you want to move the motors out.
It would be good to eliminate the wood motor mounts entirely and mount the metal motor mounts directly to the top beam. This requires having some ability to move the beam out from the back. Making this adjustable is pretty easy once you have it out at all.
- Plywood quality
The ‘wing’ frame uses the plywood as a structural element. The ‘top beam’ approach eliminates most of the stress, but you still have the legs attaching to the plywood. With just a little framing, the plywood can cease to be a structural element. I can be the wasteboard directly, or at least be shrunk down to a 1/2 cheap plywood