I'm playing as a currently 4th level wizard with the cantrip message. In pretty much any situation where any of the other party members go ahead alone, to scout or the like, or in situations where being quiet is necessary, I always tell the DM that I'm pinging the other party members with message once each minute to check in on them. While I'm certain this is allowed RAW, it feels a little bit cheesy, and I'd like to craft a magic item which allows the other party members to cast message by themselves during our next downtime.
How could you design such an item using the rules from Xanathar's and the DMG? What rarity would it be? How much would it cost to craft? Ideally it would not require attunement, and would not be very limited in uses.
Seeing as this just replicates an ability I already have, I don't imagine the DM would take much issue with it, as long as the costs are reasonable.
Best Answer
Ask your DM, they'll need to homebrew this
Unfortunately, I don't think we can help guide you here. Whether or not your DM allows crafting or allows players to come up with their own magical items is something that is independent and determined by each DM. This is also a homebrew item and not something existing that can utilize some optional rules.
There are just too many questions that we can't answer here. Only your DM can determine if they allow crafting, if those things are available, if the magic item is reasonable, all of that your particular DM is going to have to determine - including what costs associated with crafting them may be. Xanathar's does cover some optional rules and guidance on downtime and crafting of magical items, but those are purely optional and dependent on the DM to approve and use.
However, those rules still assume non-homebrew magic items. A DM would still need to homebrew this, and we don't homebrew for people here (but we definitely can assess a homebrew!)
Problems in design
I did want to add a couple notes on your design needs/thoughts. Message itself is still limited by a range of 120'. In my games, when the scouts go ahead, they are generally further ahead than that as within 120' may still be visible by those you don't want to be seen by.
Something your DM will need to consider is that granting these magic items to allow the party to all communicate in this way is basically providing a free partial feat. The common methodology to allow a creature to cast a cantrip that normally couldn't is via the Magic Initiate feat which grants two cantrips and 1 1st level spell. Creating a magic items that gives everyone this cantrip needs a determination if that balances their game.
You're not replicating something you already do
Currently, in order for communication to work, you need to spend an action casting message. It's entirely on your action/turn. These homebrew items would allow your compatriots to initiate communication - which is not something you are currently doing.