I need to " delegate _acme-challenge.localhost.example.com to an IP which is reachable by the Let’s Encrypt servers"
I have the server, it’s just hover (my registrar) won’t let me make a glue record with an underscore in it. I’m only pretty sure that “glue record” is the thing I need to make. The “Add a Record” form doesn’t have a “NS” option.
Any tips? Can I make letsencrypt use a different NS name, without an underscore? Or go about using a different method?
That might be best. Some ACME clients have fully automated integration with the APIs of different DNS providers. You can install the client, configure your username and API key, and create whatever certificates you want.
I looked at the clients I usually check, and didn't see Hover integration in any of them.
I'm not sure I want to shill particular DNS companies too much, but some of them are free, or have free plans, or are paid hosting companies or domain registrars that provide DNS at no extra cost (which may be of less use to you).
Edit: While you can always combine different DNS providers, if you aren't strongly attached to Hover's DNS service, switching entirely may be the simplest choice.
Edit again: This post was largely a digression, since you asked for help with your ACME client, and I kind of suggested switching to a different client. I'd suggest keeping your options open and doing whatever you prefer. Which might mean switching clients, or continuing to use your current client and switching to a DNS provider that lets you create NS records with underscores, or something else.