"Anybody know which HKS speed loader fits the new Cobra/King Cobra revolvers best?"
The new Cobra and King Cobra use the same size cylinder as the older Colt "D" frames like the Detective Special.
The HKS "DS" model is a perfect fit, but since the Colt cylinders are almost exactly the same size as the S&W "K" frame an HKS "10" also fits.