We are looking for the best practice for our solutions, one of those is the best way to implement meta transactions, where can we find a trusted, audited contract example with proper documentation?

To integrate meta transactions, you can go with two options. One is the existing relayer solutions and the second is to set up a custom solution.

These are some relayer solutions:

Biconomy - https://docs.biconomy.io/products/enable-gasless-transactions 

Gas Station Network (GSN) - https://docs.opengsn.org/#ethereum-gas-station-network-gsn

Infura - https://infura.io/product/ethereum/transactions-itx

Gelato- https://docs.gelato.network/developer-products/gelato-relay-sdk

You can visit their documentation for the integration process. Openzeppelin provides prebuilt libraries to integrate meta transactions in an easy way. You have to set up the environment by installing some npm packages and dependencies.

Visit openzeppelin docs, they mention all steps to follow. Here is the Openzeppelin documentation: https://docs.openzeppelin.com/learn/sending-gasless-transactions