I think the best method for a simple blade cover would be a fold-over or taco style.
Get a long narrow piece of kydex, that when folded in half will cover the sharp edge of your hatchet. Heat @ 300 degrees for 3 minutes or until the edges ever so subtly begin to curl, and then fold in half over the sharp edge.
You can get by with minimal tools on this, by using a couple of towels folded up as a "mold," and place the hatchet with kydex into the middle of the towels, and sit on it for a good 5 minutes. (The towels will help mold, but also make the kydex cool slower due to insulating it.)
Then once cool enough that the kydex is molded, remove from the hatchet, drill a couple holes, and install rivets or chicago screws. Drill an extra hole, run some elastic shock cord through, and use to secure the kydex over the sharp edge of your hatchet.
Hope this helps!