SP Connect SPC+ to Quad Lock mount adapter
An adapter disc that lets a phone in an SP Connect SPC+ case ride on Quad Lock bike, motorbike and car mounts, or the other way round if you flip the description. One face carries the four-lug Quad Lock style boss, about 37 mm across and 4 mm tall, and the other reproduces the SPC+ twist interface with its shallow bayonet ring and detent recesses. The 6 mm thick disc prints flat with 0.2 mm clearance on both patterns and a 1 mm chamfer so the lugs start easily.
Both brands revise their interfaces and neither publishes drawings, so a downloaded adapter is a gamble. In SpeakCAD you measure your own case and mount, describe both patterns, preview the adapter free in the browser, refine the twist tension in the chat, then download the STL and print it in PETG or nylon.
Create a 45 mm diameter, 6 mm thick adapter disc with a Quad Lock style four-lug boss 37 mm across and 4 mm tall on one face and an SP Connect SPC+ style bayonet ring 40 mm across with two 6 mm detent recesses on the other, 0.2 mm clearance and 1 mm chamfers.