The following is anticipating using a folder about the same 4" handle length as a traditional sgian dubh. If you went with a longer folder it would be possible to mold a sheath to hold the folder about 1/2 way up the handle leaving about 1/2 the knife above th hose to grip. A 4" handle though would only leave at most 2" visible and available for gripping above the hose so your looking at a folder at least about 5" long folded.

This has been a thought I've played with for quite a while and I always come back to the same outcome, it's not practically possible to wear a short folder in the sock and have it serve as a sgian dubh without sacrificing the decorative value, easy draw/use functionality or the secure stowage.

The sgian dubh is mostly a decorative knife worn usually for appearances so the handle needs to be long enough to stick out of the sock to be seen. To anchor a knife with a handle that long into your sock, you will need something running down into the sock at least the length of the handle (typically the blade in a sheath). A folding knife would eliminate the anchoring portion in the sock and increase the weight in the handle above the cuff of the sock making the knife extremely top heavy and prone to wiggling free of the sock while walking.

I carry the "Officer" sgian dubh by Gaelic Themes which is sort of a folder. The blade is fixed but there is a cork screw and bottle opener/screw driver folded into the handle. It's a very top heavy sgian compared to most.

The best solution I've found for anchoring a top heavy knife into a sock with minimal anchorage in the sock is to replace the stock sheath (far left) with a molded sheath (middle).



The extra height on the sheath wraps around the bottom of the handle anchoring it into the sock very securely.



The molded opening also protects the expensive kilt hose from being sliced up by providing a constant opening to easily unsheath or resheath the knife.



The sheath rides right at the top of the cuff so it's very low profile.



So far I've not come up with a design for a folder that can be worn in the sock, stay secure in the sock when walking, be decorative / visible and not be 3 times as long.

Even a sheath that gripped the folder 1/4 to 1/3 of the way up the knife might still have problems with wiggling loose without a bit of blade running further down.