Here's a faster and a more simple solution:
Use a simple plane for your emitter geometry and mask it like you need.
Attach it on the back of your light fixture (slightly inside) and that's all.
Make sure the emitter's normals are facing in correct direction.
Now, if you make the material of fixture (I mainly talk about the inner side) made of pure black,
it will not cast indirect rays from the spot and the result will look like this:
If you use a lighter fixture color, the emitted light will start bouncing inside the fixture and
it will start to look more interesting. Although, I suggest using high rough single BSDF material
for the inner face of the fixture. That will add a lot of speed.
Controlling focus is as easy as changing the shape of the fixture. In this example, it's more long/narrow.
I'm sure one can add more complexity in this basic setup but,
it's not always good adding more detail if you're also after rendering it
fast.
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