1) Hash out an agreement or whatever in a public thread on reddit (this only works for people in civcraft with well known reddit accounts). I can make a special contracts subreddit if it's convenient. Or wherever, but it needs to be visible to a web bot without being logged into reddit.

2) Go to https://archive.today/

3) Type in the reddit website address. Note: I suggest avoiding the use of "context" or single comment "permalinks" because people later on might object to the missing context as potentially misleading. Whole thread is best, and do it all in a fresh thread in few comments so there aren't any "load more" links or anything that could be hiding critical context.

4) Archive that shit.

5) You can now post that link later, and have a virtually incorruptable, time-stamped, comment-deletion-proof, non-retractable, third party held record verifiable by anybody of an agreement made, with game identify backed by your entire reddit posting history. I wouldn't stake a year's in-game savings on it, necessarily, but quite a lot of trust yes.

You could even do private contracts that nobody knows about until/unless it becomes necessary to drag them out in a dispute. For that, you wouldn't want to use a general purpose contracts subreddit. You'd want to make your own sub that only the other guy knows about, archive it, then delete the thread/sub and hold onto the archive link. I believe the archives are searchable, so privacy isn't rock solid, but it won't get any casual attention.

There are also lots of cool convenience features outlined here https://archive.today/faq.html such as hashtags to auto scroll to relevant parts of a page (like if you wanted to point out a clause in a contract in a comment)

example https://archive.today/ffgDQ

edit: if you want even more security, I imagine there might be a way to use the same concept with minecraft's curse forums using your actual minecraft account so as not to rely on known reddit<-->game aliases. But with the new account name change option, I'm not sure if that works well.