If the dimensions are right you might be able to get a nylon sheath that is compatible to mounting setups for military equipment. If you are set on making a sheath, which is a noble effort, you might consider making one with Kydex which is available in tan, black or OD green. It can also be spray painted for a temporary way of blending with the surroundings.
Are you thinking of some sort of set up like a diving knife? The calf and shin area is difficult to attach equipment to for a couple of reasons. The shape of the area makes things prone to sliding down and around during activities like running, climbing, crawling and jumping. The legs are a part of the body the move the most.
There is nothing to anchor it to such as a belt or loops and such. Things like knee pads (which are relatively light compared to a knife) are hard enough to keep in place during periods of high physical activity.
There is the option of attaching it to a boot but that has its problems too. If it is in the boot it could be uncomfortable. If it is outside the boot it could be cumbersome, and without a super reliable retention system the knife could be lost.
Is this carrying option by request of your friend? If not, I would suggest a sheath that is MOLLE compatible. That way he could mount it to a ruck, vest, LBE, sub-load (that would allow him to wear it on his thigh area) or anything that has PALS webbing. The set up that will allow for the greatest number of carrying/mounting options would serve the best, and it would be modular. Often what seems like a good idea in thought does not always work out or is totally unacceptable in practice or by regulation. It is generally best not to attract attention in the military, and and odd carrying method for a custom knife might do just that.
Check out Tactical Tailor for their "Malice Clips." They have holes 1 1/2" center to center that can be attached with screws and aluminum posts through 1/4 inch rivet/eyelets on the sheath. Another option are TEK-LOKs available through KnifeKits.com for a handy way to mount the sheath to a belt. The holes on the TEK-LOKs are also 1 1/2 inches center to center and they come with the screws and aluminum posts.
www.knifekits.com
www.tacticaltailor.com