Happy new year to all.
Very interesting and nice job, Warren. Thanks for sharing.
In your explanation for the material choice, you mixed up price of cast and extruded PMMA (plexiglass is a trade mark, PMMA is the right chemical designation). Extruded PMMA is cheaper: it's a continuous process, at the opposite of casting.
Both are more expensive than polycarbonate here.
They have better optical properties than polycarbonate: transparency is higher (but polycarbonate has the same transparency as glass, which is not bad at all), it can be polished, but the most important thing is they don't yellow with time and UV.
For machining, PMMA is the best material, especially cast PMMA. But you're absolutely right for thickness variations and the Vcarving issue, unless you have a machine with a floating head. Cast PMMA gives excellent results with etching techniques, in this case with a pressure tool you don't care of thickness variations.
Why are you concerned by fire rating with LED lightning? Polycarbonate is way better than PMMA in this domain, but it shouldn't be an issue in this case.
W7 - Aspire 8.517