All depends on how much water it needs to hold really. For something like bettas that doesn't need to be very deep 1/8"-3/16"(3-5mm?) would be plenty. 100% Silicon would be the sealant. Make sure it's the long cure type as the quick cure is not aquarium safe.
Advice would be glass and acrylic tanks are built differently so if you're going off of youtube make sure you watch the right ones