You have not really given us enough information to suggest the best solution. For example, is this because you host many websites for other people who all use your custom nameservers? Either way, if you are hosting many sites with custom nameservers, look at what AWS does for its nameservers for route53. Theirs are set-up like so:
ns-1271.awsdns-11.co.uk
ns-522.awsdns-02.net
ns-433.awsdns-03.com
ns-1870.awsdns-03.org
In your case, you'd likely want to each domain used in your custom nameserver configuration to be registered at a different company. In this way, you no longer have a single point of failure.
ns-1271.awsdns-11.co.uk
ns-522.awsdns-02.net
ns-433.awsdns-03.com
ns-1870.awsdns-03.org
In your case, you'd likely want to each domain used in your custom nameserver configuration to be registered at a different company. In this way, you no longer have a single point of failure.