How edit html pages for Ingress-nginx default backend on kubenetes

I have installed nginx-ingress via the 1-click process here on DO to my cluster. Everything is working flawlessly. In the helm chart values.yaml file there is an option to enable a default backend pod/image. This works as expected. What I can’t figure out is how to edit the html files inside the prebuilt image. For example 404.htm simply says “404 error” I know I can create my own custom backend image for this purpose but I am trying to stay with the helm chart resources as provided. Here are the references to what I am working from:

https://marketplace.digitalocean.com/apps/nginx-ingress-controller

This is the nginx docs I am following,

https://kubernetes.github.io/ingress-nginx/user-guide/custom-errors/

I have tried to exec into the pod but there seems to be no shell available for me to look around inside the image. I can’t find any git repo for the image so I don’t know what’s in side. The image spec’ed is k8s.gcr.io/defaultbackend-amd64

Any thoughts, or pointers to documentation on how to do this would be appreciated. I have posted this question on slack and at the git repo issues with no viable answers.

thanks.