Whatever external storage solution you use it will be slow and unpredictable since it has to go through the internet. You could use S3 or some other service directly in your clients web site if they're willing to modify their application to do that.
The simplest solution is charge the customer the additional cost for running a larger node and dedicate it to them. If they need 60GB of space then they probably don't want shared hosting

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Paid supportHow to ask for help
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