If making your own caps ends up being the best solution, TPU (thermoplastic polyurethane) is probably your best bet. I don't have specific information on printed TPU filament and ozone, but TPU is widely regarded as one of the most chemical-resistant materials you can easily print with, and this page by Ozone Solutions rates polyurethane (no mention of specific types) A/Excellent described as:
Ozone has no effect on these materials. They will last indefinitely.
Being at least slightly flexible, TPU will also yield a good seal without any additional gasket. You should probably choose an unpigmented "natural/clear" TPU filament in case the pigments do react.
It might (probably would) also work to coat an existing cap with polyurethane. I'm not sure how you'd best get it to adhere, but lightly sanding the plastic then using a spray in multiple coats is what I'd try first.