Proper way to change proxy_buffers for a single host?

[dokbua]

Using nginx-proxy-manager in front of mailcow-dockerized, and after the latest update the SOGo webmail interface required an increase to proxy_buffers - details here. Also, a modification to the root location directive is required.

https://nginxproxymanager.com/advanced-config/#custom-nginx-configurations suggest such changes should be done to snippets under /data/nginx/custom, but the explanation is a little sparse and I've not been able to locate any examples anywhere.

Is there a kind soul who briefly can explain how to make this change?