I need to power my canon speedlite 600ex-rt. It has an external power plug for which I can find plenty of battery packs (e.g. Godox). But I just want to plug it into 240V AC supply voltage. I cant find info about what type of power it takes in through the external power plug (apparently it is a quite high voltage, not a usual 4-6V (4xAA)). If I knew that I could make something myself. I have the correct plug.
You have two options. There is only one commercial product for this that I know of, from Innovatronix. I looked into this years ago as a potential continuous power source for doing bird photography off a deep cycle battery (mainly to improve recycle times, less so for longevity.) The productis the Tronix SpeedFire...honestly not sure if it's still sold.
The other option is to make your own mains power supply adapter. It actually is not all that difficult, and this article covers the specifics:
The latter is what I eventually did, although I have not used it in some time (I generally just seek out birds when the sun is out now...hell of a lot easier than trying to lug around a big, heavy deep cycle around with me all the time.) This is pretty easy to get working off a deep cycle or small sealed lead acid batteries. You would need to build an AC/DC converter if you really wanted to get it working off of an AC power outlet, and that's beyond any of the articles I've found on the subject.
The official Canon power pack normally is powered from 8 AAs, but there is a capacitor charging circuit in the battery pack itself. You might have the best luck getting the official pack and then supplying power to the hookups in it since those are well known and would make use of Canon's charging circuit in the pack.
Keep in mind that the external battery pack connector does not provide power to the electronics of the flash, so it is still necessary to provide power to the flash itself. I'm not sure how long a set of batteries will run the flash without having to drive the flash bulb, but it may still become an issue during the length of your exhibit, so you may also have to wire up an alternate DC power supply to supply the power that would normally be provided by 4 AAs inside the flash as well.