First, they have to meet the points collected in: What makes a relay trust-worthy?
Second, they have to publish a clear and comprehensive data policy like ours: Data Privacy Notice | Flashbots Docs
Then, I liked what Lido did on their call for relay providers asking the operators to introduce themselves and explain how they plan to contribute.
With that information I think the product team at Flashbots can make a decision of which relays can support our projects, starting with one or two to experiment slowly and check the effects.
Who are the relays interested in receiving our blocks?