I got custom whitening tray from my Dentist and they work great. My wife ordered them from Amazon, which is exactly the same as what I got. https://www.amazon.com/gp/product/B004EKVOU2 Custom trays are nice. You just put a little gel in the tray and wear them for 10 mins. The gel is cheap and the trays will last forever.

I have custom trays made from my teeth. Works awesome. I got it off amazon. https://www.amazon.com/Upper-Custom-Dental-Whitening-Bleaching/dp/B004EKVOU2

Try custom whitening trays. I love them. You can order them from your dentist. They take an impression of your teeth and my trays that hold the gel to the teeth. The trays will last forever if you take care of them and can just buy new gel from any where. Even use Opalescence (mentioned). I got my first pair from my dentist and then lost them moving. I got my second pair from Amazon. https://www.amazon.com/gp/product/B004EKVOU2 I have been really happy with them.

You can just order whitening tray like the dentist would make you online now. There is a few different places online. I got mine from Amazon. https://www.amazon.com/gp/product/B004EKVOU2