I just use a simple little device that i paid about $4 for that pushes down on the button on the side of the MK II magazines to make for quick and no sore thumbs loading. It only takes a few seconds to load the magazine this way.

I think if you were to somehow make a square hole in a block of wood slightly larger than the outside dimensions of the magazine, set it on top of the magazine button and push down on it as you load, it might work nearly as well.