Hmmm, I would usually go for ASA for anything outside, but searching around for transparent ASA comes up empty... I guess maybe transparent ASA isn't a thing? Too bad, since you could smooth it with acetone to make it even lower opacity. You could also look at Polycarbonate and PETG, which I know can be made transparent, but those might yellow a bit after a few years.
I'm interested in other people's thoughts. Definitely a good question!
Oh, another thought - you could use a UV blocking clear coat such as this on your print. I still would avoid PLA though, since it might get hot in the sun. It would be important to get a complete coat with that spray, even between layer lines. You might want to think about using a clear resin if you have access to a resin printer, since it would have a much smoother finish before being coated.