Wow, that's really small. Only 43 cm3 (without the plug) for the EU version. I own an a Anker Nano II 65W GaN Charger and that's already small with 64 cm3. I can imagine that Rolling Square one getting even hotter with that low amount of material, and thus cooling.
It's not actually made from a single soft plastic, the inner lining is soft, and the outer parts are quite hard. I don't think there's a non-plastic mouthguard in the market.
Last summer, I (33m) was diagnosed with mild sleep apnea and began treatment with a mouthguard. It took me almost no time to get used to sleeping with one. And it had a great effect on my sleep quality, but lately, I've been experiencing more and more trouble with my jaw. Considering switching to a CPAP soon.