Yes, PLA creeps ridiculously fast in hot conditions it is unsuitable for use in any environment over 35C tbh.
PETG is better but will still warp/fade over time if it's exposed to the sun, and likely need replacing.
You need to be printing in ABS or ideally ASA if you want all-weather resistant cases. Which means now your printer needs an enclosure and ventilation...
I printed a sprinkler to stick on the end of my watering can out of PLA.
Its been outside in all weather, including direct sunlight, for three years now. And water runs through it almost every day for 8 months of the year. It's a little faded, but that's it.
IMO the things people say about PLA regarding exposure are vastly over stated.
It can survive fine when under no load. It probably fades color less than abs does actually. However When experiencing any kind of mechanical load - say, a tablet squeezing at the edges for a friction fit - it fails rapidly at elevated temperatures.
I don't think your particular case would have any creep as it's not mechanically leveraged in any real way.
If you were to print something like a cupholder for a stroller or bike where it's holding something up with some weight while in the heat is where you would notice it especially with repeated impact. Most likely wouldn't outright fail but under constant load you would notice it starting to bend a little. That said you can absolutely over engineer it to prevent that rather than switching materials which can be a huge pain depending on the printer.
I did my hydroponic tower in PETG but it honestly would've been ok in PLA since it's just a static fixture. I've had a PLA badge on my car for 3 years in 110+ summers and similar to you I've only noticed fading on the raised white lettering.
I think it depends on the usage. If the size/shape doesn't matter or is mated to metal, then PLA is more than fine in colder climates (i.e. not Texas), but as soon as the average temp of it's operating environment is within the Glass Temp range then PLA isn't good.
In your sprinkler's case, the water is chilling it bellow glass temp when operating.
Yeah ABS is super finicky to print, a temp controlled enclosure is mandatory and you often need perfectly prepped full adhesive plates with large print brims. But it's definitely the only true weather-tolerant plastic if you can get it to behave.
Finally, something 3-D printed for bibliophiles. You will make money with this as a niche, selling cases to people with older Kindles, for example. It's very difficult to find a protective case for any of them that are more than five years old, IMHO. Bless your face!
Though you are absolutely correct. I've made a universal base with replaceable arms (since that's what kept breaking on my wifes). All I need to do is fit the arms to the eReader and figgure out where the sleep magnet goes and bam, new eReader case.
Currently I've made cases for the Onyx Book Nova 3, Kobo Aura One, Kobo Nia, and Kobo Clara 2e. With plans to tackles anything I can easily get my hands on.