How to protect origin server on Google Cloud that are behind Cloudflare?

I have a some DNS A records pointing to the public IP addresses of some Load Balancers on Goolge Cloud Platform. The domains are protected by Cloudflare but is there a way for me to protect them in case their public IP addresses are exposed somewhere? I need this mostly for compliance reasons.

I'm thinking of using Google Cloud Armor (https://cloud.google.com/security/products/armor?hl=en) to only allow Cloudflare's IP addresses but people can still hit my load balancers using Cloudflare Workers and Cloud Armor seems expensive even with pay-as-you-go. Is there a better way to do it?
Google Cloud
Google Cloud Armor is a network security service that provides defenses against DDoS and application attacks, and offers a rich set of WAF rules.
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