I think just replacing the tub with one that's not opaque is the easiest, best & most cost-effective option. Plexi is pricey, plus the work to install? I'd replace the tub.And it doesn't need to be completely clear either- it should just let some light in, with a fogginess that gives privacy at the same time. (Sterilites do that.)
BPs only need enough daylight to know it's day or night- aka "photo-period". You can take them outside occasionally for REAL sunshine that will benefit them, but they're mostly nocturnal so that constant sunlight (or artificial lighting) is not natural for them anyway.
Keep this tub for a side option when you need to clean...
That's the most important thing.