How far are they going to go to validate it’s legit if you make one up?
But say you don’t want to forge one for just any company because that might not be legal. You could make your own company with a site and make offer letters that way.
All you say in this case is that "the actual offer letters are proprietary information of the company that I'm applying to and I'm not at liberty to share them" or some similar verbiage.
FWIW I've done this several times (multiple offer scenarios where I have companies compete against each other for me) and never once has a company asked to see actual proof of the other offers on condition of hiring me. If that was a condition, I would say, sorry, I'll go with one of the other ones