My initial plan for the window is:

• Cut a few circles out of some float glass,
• Clean them thoroughly with IPA and HCL,
• Build a stack of them in the kiln of the desired thickness,
• Fire:
  • Slow ramp to 675°C,
  • Hold for five minutes,
  • Fast ramp down to 560°C,
  • Hold for 10 min,
  • Slow ramp down to room temperature.
• Polish,
• Harden in a 400°C bath of potassium nitrate.

I had good success tempering bottle glass with a 560°C soak in the glass sphere project.