If these are "private" servers, you may find using the DNS-01 challenge more suitable, as then only the DNS server needs to be exposed to the world rather than also exposing the web server. The only tricky part that that you need to automate the proper response to the DNS challenge, either through using a DNS server that supports an API (and a plugin for your ACME client to use it), or through something like acme-dns or agnos.
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