90° adapter with guide for corner clamp

Quick and dirty remix of the very well designed corner clamp from jtronics.
Just merging the guide of the central part with the 90° adapter b part.
Improves the rigidity to the overall clamp.
Fits easily; not loose but could be made a bit mode tight.

Only the 90° adapter is available, feel free to make your own adapters for the other ones.