A post is better. The moon-clip is primarily damaged by crushing the cartridges in rather than pulling the tips apart. So while a speedloader pouch does offer some protection, what you really want is a post that fills the center gap. Also, with a moon clip in the pouch, you have to stick your finger in the clip's hole and pull it out with one finger and then readjust your grip. Speedloaders should not be gripped by the release, but the outside diameter of the speedloader body sticks up high enough out of the pouch to get your fingers around it. The moon clip package is shorter and sits too low in the pouch.
So my suggestion is a post or two posts that the clip(s) can slide onto with the post sticking through the hole in the clip. It should have a step in the diameter so the clip rests on the step. Having two joined together at the bottom helps keep them from tumbling sideways in a wide pocket. You can also sew the pocket narrow enough that one won't tumble, but wide enough you can still get your fingers around it in there. Having two joined also helps prevent the post coming out of the pocket with the clip.