Di2 dream bike:
I have the older Di2 with the junction box that has the charge port. It's mounted under the stem. Now, there's an alternate configuration,with the charge port and status lights in a bar end plug. I suppose it needs compatible handlebars to run the wire, or perhaps it can be under the bar tape?
For the junction box version, I got this
K-Edge mount. It's less than 1mm thick and sits on top of the steerer spacers. A lot nicer than the Di2's plastic strap that wraps around the stem.