[schoen]

The cryptography world has a ton of ideas about this, but it's possible that most of your customers wouldn't find it easy to understand why some of those ideas are correct and fair.

A simple one (not necessarily anywhere close to the best that cryptography people have come up with) is to combine several sources of randomness in a prearranged time order and format, and use the result as input into a prearranged cryptographic hash function. At least some of those sources should be publicly verifiable, and at least one of those should be https://beacon.nist.gov/home. I can think of critiques and limitations in this approach, but it's a good start!

Edit: someone elsewhere in this thread has given a link to a more sophisticated method.