You don't need a diffuser or a lens. A soda straw will work fine to exclude extraneous light.

Measure the light level at the desired time of the day with a light meter.
I __assume__ you can get somewhere in the vicinity close enough to make the measurement, since sunrise covers a large area of sky.

Then set up the light meter in broad daylight, pointed at the sky (with a snoot to limit its field of view and exclude the sun) and read the light level. Put a combination of ND filters over the snoot until you get the same reading as sunrise.

Place that filter set over your sensor, pointed skyward under the same conditions, and use that level to calibrate the trip point.

- Leigh