I decided to use the steel wool and large particle Mg, but to add some Mg/CuO thermite to give it a bit more heat and spark.
I started by cutting off a thin slice of the steel wool, then cut off an even thickness piece of the first slice.
I'm using 18 gauge solid copper wire, crimped over the ends of the steel wool. It's important here to get good contact of the copper to a large area of the steel wool. Not enough steel wool in contact with the wire causes the steel wool to burn off at the crimp, which may lead the igniter to fail. I'm sure you could solder the steel wool to the wire as well, but it's working fine by just crimping.
Next the steel wool coated in the larger Mg by dipping in the container.
Finally 1.3 grams of thermite is added to the finger of a latex glove and taped to seal it on the lead wire. This thermite mixture is 75% black copper oxide and 25% 1000 mesh Mg. I'm sure other thermite formulas would work about as well, I also doubt the Mg has to be this fine of a powder, something in the 300 mesh range should work as well.
Here's is the moment of ignition of the 1.3 grams of thermite.
1/30 of a second later you can see the nice amount of sparks thrown.
Another frame later (1/30 second)
Next frame you can still see Mg particles burning.
This picture was 4 frames after the last picture, so the duration of the burn was in excess of 1/4 second.
Click Here for a short video of the test.
For the next test I wanted to try igniting an old KNSU grain.
Here's the moment of ignition.
This is 1/30 of a second later.
Next 1/30 second later.
Fast forward 4 frames and you see good ignition of the KNSU propellant.
Fast forward 8 frames and the propellant grain is in full burn. The rocket would be off the pad at this point in less than a quarter of a second.
Click Here for the video of the test.
I would caution anyone wanting to make these igniters. The Mg thermite mixture burns with a very intense white light that could damage your eyes. Don't look directly at any Mg fire. This thermite formula is fast, so fast that it pops when ignited. Large amounts of this thermite would throw heat and burning Mg for quite some distance. Use extreme care and only use this in the lowest quantities practical. While thermite produces very little expanding gases, it does create thermal expansion and could cause a sealed container to explode, again, use extreme care!
I'll perform more tests and post the results as I go. But from these early tests I see no reason why this can't be an adequate if not better option for igniters than standard pryogen formulas. It also should be easy to scale this to most any size motor, excepting perhaps very small motors.