SFC Smallville has a bigass knife (pictured bottom) which had a fairly sturdy kydex sheath with excellent retention–mated to a cheap nylon web belt loop (center). Why someone would do that, I do not know, but it was probably flimsy coming out of the factory and was fraying badly when I saw it. Clearly, it needed something a little more stout (top, riveted to sheath).
In my original review of the Boker Cop Tool, I sang its praises roundly, except for the sheath, which really seemed like an afterthought. No retention, iffy belt clip, and it sat too high, so the top stabbed you in the side. Using a scrap from the ammo pouch, I fixed all of these issues. Because my newer Boker has a glass breaker on the top, I incorporated that into my retention. Not quite as pretty as I would like, and several design choices were based on available materials rather than what I wanted, but I am pretty happy with it for now. I have wanted something like this for years.
Also, that gouge on the front was put there by the cow, not me. This is a rough draft; it’s okay.
[Update: Fixed picture and added link to old review, other minor additions.]