When ordering the LongMill, I decided for a different router (Mafell FM 1000 Fresatrice elettrica 1000 W, since 4000 a 25000 giri/min : Amazon.de: Business, Industry & Science), not knowing that this required modifications on how to mount it to the Z-Gantry. It’s 43mm mount is not a real issue (holders are relatively easy to find: 2EXTREME Milling Spindle Euro Neck Mount 43 mm with M6 Thread Spindle Mount Euro Neck CNC : Amazon.de: Musical Instruments & DJ), however because of the different shape, it is mounted much closer and higher on the gantry. Therefore using the original steel plate, the bit could not reach the spoil board in some cases.
As I stumbled across a message in the LongMill Facebook group (https://m.facebook.com/groups/mill.one/permalink/1305379869933341/?m_entstream_source=group) that seems to mention the exact same issue, I decided to post my custom holder here because it might help someone. The original mounting holes are identical so that one just needs replace the steel plate (screwed to the anti-backlash nut and the linear glides) with this new design (I used 12mm Aluminium which seems so work fine).
I don’t like to use Facebook, but maybe Felix who asked the question in the Facebook group is also active here in the forum 
NewHolder.dxf (37.5 KB)
) and also, I wanted to place the router lower then I could with the new gantry plate. This was initially not intended, but I did not want to make a thicker wasteboard for the new setup.