On a custom Hebie carrier for an older Bernds folding bike there were M4 threaded rivets. A good initial position for an adapter. The mount has a 5mm wire design with holes of 4 mm (rack side) and 4.5 mm (backlight side). The faces on the rack side are angeled and the backlight holes have a distance of 50mm ( common for most lights, in this case Busch & Müller ). For detailed geometric information download the file. The stainless steel version works fine to date. A little bit of flash rust started. The 4mm holes had to be reworked (to tight). Furthermore the cheaper Alumide version makes a sufficient impression too (not tested).
The 3D Model was built in SolidWorks2012 (Dassault Systèmes)