Festool bluetooth adapter, good quality housing (an impact and appropriately IP rated type), socket, your time and accepting the risk of making a DIY circuit to interfere with mains voltages. Not without costs of all types.
I get the appeal for some, but it's not really a viable option for most to make one themselves. In my case and despite having the equipment and experience to assemble one, I simply sold my older Midi and bought a new Midi I with bluetooth, for less cost to change than it would have been to buy the parts for an ugly adapter (sorry, but it's true) which would be prone to damage or failure.
It's most likely that Festool don't make one because they don't want idiots triggering saws or other cutting tools using a bluetooth remote or battery using something with their name on it. When it's a proprietary form factor, designed to interface neatly in a suitable CT or via built-in BT switching, it's pretty safe. You can't fire up the tool with the sharp ends using the remote, only the CT itself, so it's a non-issue in that case. But people do really, really stupid things. Germans don't like that, generally speaking.
And as for supplying one to fire up other brands of extractor/vac, well, what sort of sense would that make to a company who produce and sell their own line? Even getting over the risks of enabling idiots, it makes no sense at all from a commercial perspective.