At the outset, I'm using a sheath that was holding a SAK and the RSK Mk1 fits perfectly:
http://www.swissknivesexpress.com/loccorbelpou.html#It's black, vertical-only, velcro, lightweight, and not at all stiff. It's only $5, and may not withstand heavy use & abuse, though I don't have the experience to imagine what may cause it to fail, or what creek I might be up if it does.
Blitz' suggested SPEC-OPS Super Sheath ($20) seems to be of substantially heavier construction, bulkier, but since it is nylon, I don't think its weight would be noticeable.
And I gather that kydex is moldable plastic that would offer stiff protection but which would be almost double the width of the knife once the grommets or screws are in place to hold the two layers together. I could see where kydex would be very useful, however once molded, a kydex sheath couldn't be used for other equipment, whereas my el-cheapo sheath is flexible enough to fit other knifes or my backup headlight.
I took a look at the Bianchi Accumold 7303 Single Mag/Knife Pouch w/ snap closure, but I think I'll end up poking around the next few months until I find one that I'll ultimately dedicate to the RSK Mk1. I'll post an update if anything changes.