Just a word of caution though, Perspex (PMMA) is terrible with impact resistance.
It will shatter like glass if struck by any stones or debris.
It will most likely absorb enough energy to save the light, but it will be a 1 and done kind of a deal.
Rather get your hands on some clear Polycarbonate. Maybe a 3mm.
It's the same stuff aircraft windows are made from.
Sometimes it's referred to as Lexan, but that's just a brand name.
A place called Maizeys will sell you some off cuts. They have branches all over.
I'm a design engineer for a plastics company in PE, and we often make helicopter and small fixed wing aircraft windows from Polycarbonate.
The stuff is as tough as nails when it comes to impact resistance.
The only downside is once you scratch it, it's impossible to buff out the scratch.
Side note :
NEVER clean any acrylic plastics (Perspx, Polycarb, PETg, etc.) with any type of solvent based cleaner. Only use mild soap and water.
Because the plastics are petroleum based, solvents will cause the plastics to attempt to revert back to their base components. The result is what is termed 'crazing'.
The plastics will first become milky/dull and then show tiny hairline fractures across the entire surface.
The only challenge you'll have with Polycarbonate is temperature when it comes to making the bends/forming.
Polycarbonate is a bugger to bend by hand.
Sometimes if it is old material, it will draw moisture when it stands, and when you overheat the material the bends will be full of bubbles from the moisture 'boiling' inside the material.
The best solution is to put the raw panel of Polycarb in an oven on 120 degrees Celsius and you'll leave it in for 3 hours per millimeter. So on 3mm material it will be 9 hours on 120 degrees C.
When you heat it to bend, make sure the heating is done evenly at about 140 degrees C.
Only the material starts to become flexible (test by bending slightly by hand) it is ready to bend.
What we generally do for molding to parts like light lenses and windows, it we make a fiberglass copy of the part.
You could even make a paper mache copy over the face of the light. Once it dries, smear some body filler over the low points to fill them up and sand the face completely smooth (any high or low points will show in the plastic), then heat the whole piece of material and drape it over the mold and let it cool.
The plastic will have the exact shape of the light it covers.
Hope this all helps.