Even if you find a safe resin.

Might there be microscopic layer lines? Or other unknowns you're not familiar with? Making 3d prints that can be cleaned is non-trivial, maybe there is a surface finish involved, etc.

Also how do you know your design is correct? Won't cause your teeth to move? A 3d scan doesn't mean you know what a mouth guard should look like.

All of a sudden, having a product that's made with a vetted process is pretty attractive -- and 600 USD seems like a bargain.

What's the cost of having your teeth fixed, if they accidentally move? (Not to mention the discomfort, which can be considerable)

I'm not entirely sure what you're trying to convince me of. I did pay the $600, I even said that it's probably a cheap price to pay to treat the apnea.

That doesn't really change the fact that it feels kind of viscerally wrong to pay $600 to pay for two glorified pieces of plastic, and a part of me still does feel I could clone it competently. I haven't bothered for the aforementioned safe resin, and also because I already have it and I have enough money to just eat the cost and complain about it.