This design is not a true spark arrestor but a spark deflector. Many tracks now require some form of spark arrestor to cut down on the possibility of claims for damages from passengers with burns to their person or clothing. In very dry areas it may also be necessary to cut down the risk of starting fires. This device will prevent most burns but it will probably make the risk of fire worse.
If you want a fully functional internal spark arrestor look elsewhere. This device was made for a rather different reason.
My passenger hauling loco is a Simplex. When I first bought it I had considerable trouble keeping up the boiler pressure all the way around the track. The oracle's were consulted and the usual conflicting advice given. In the end after several experiments the blast nozzle position and size were changed and steam production is now much more consistent. However this has sharpened up the blast and increased the number of 'robins' especially up the bank on our track. Not only that but the lubricator was managing to get in on the act and cover me with surplus oil. I had to take two pair of glasses with me to the track, one to drive the loco and a clean pair to allow me to drive the car home!
Initial investigation was on an in-smokebox filter. This was quickly rejected as being too awkward within the existing smokebox layout. It could be done but that would mean moving or modifying most of the pipework. Not a quick or simple solution. So an external solution was required. Watching the trajectory of the sparks shows that they come out of the chimney vertically and can reach 6ft or more. At that speed a mesh over the chimney was likely to break them up and shower the driver with more but smaller bits. So a solid deflector was required. An alluminium ring was turned up to sit on the chimney top, two M3 bolts act as supports and a piece of alluminium angle provides the deflector. It is very light but even the fiercest blast won't move it. The ring does need to be a good fit in the chimney otherwise it can rotate and send the sparks towards the driver. In this case it can be easily achieved by gripping the ring in a vice and squeezing gently to adjust the fit.
I have used it for about 6 years now and wouldn't be without it. It is totally effective at stopping oil and char landing on the driver and passengers. It is small enough to be relatively inconspicuous. If it has a fault it is that sparks now shoot out sideways which may not be such a good idea in times of high fire risk. (Very rare on our track!)
There is room for some experimenting with the shape since in plan view the deflector could be round. Not the rectangular shape which I ended up with. Perhaps it would look better if the ring was made of the same material as the chimney top. In this case copper.
The next project in this direction was a deflector above the safety valves to prevent their plume of steam coming straight up and obscuring my view. (I wear glasses and the steam condenses on the lenses.) Not a problem on a hot day but on a cold humid day this can really make driving difficult.