Here is a link to 3D files which can be used to print the pickup-bracket should someone want to replicate this setup. These parts have been tailored to Johannes’ instrument but I’m happy to assist with modifications.
These have been printed in PLA and PETG on a cheap filament printer (Ender-3) and in ABS (for Johannes) on a more expensive Makerbot. My advice is to go with the PLA or PETG as they print very nicely, the ABS is fiddly and tends to warp. All the parts are oriented as they were printed in their respective files. There is a 3DM file (Rhino) with the originals if someone wants to remix, it would be cool to share here if you do...
You will need an M6 bolt of about 70mm with a matching nut. For the Clamping action you need two M4 bolts with a socket cap (30mm if I remember correctly) with matching nuts. Plop the nuts down into the slit on the back of the center piece and the bolts will connect with them when assembled.
This is all fairly self-explanatory, except the two perforated boards which can be slid into a cutaway groove on the back (under the fingerboard). These are optional and intended to help you keep your components tidy, good for ziptying stuff to etc. Johannes has opted to size his preamp circuit so that it slips into the groove (nice touch that I hadn’t thought of). The Groove is 4 millimeters and can just as well be laser cut acrylic as 3D printed. There are holes on the clamps which bellwire fits nicely through, this is also to help you tidy stuff up...
There are some More versions of this including one with individual arms for each pickup. Similar parts have also been prepared for cello (see this project). Happy to share those as well.