Greetings. I have struggled to find a suitable recessed outlet box to buy in the EU, so I have designed one myself. Box dimensions are 110x110x62mm, and inside it can fit a 86x86mm power outlet with its decorative cover of up to 11mm thick, if you want it to stay flush.
The hole fits a 68mm diameter round electrical boxes, you can choose the depth of them to fit your needs - I went with the 40mm deep one.
I suggest you print it in provided two-piece design and just glue the front cover, because you lose lot of material and add a lot to the printing time because of the supports. But if you do not care, there is a single piece design available to download. Personally I used the box for a wall mounted tablet dashboard, but also works if you just need a deeper power outlet in the drywall. Have fun and thanks.