Just un update about my calibration panels.
This time I've done something bigger than the previous structure. This new 90° angle is done for big objects.
The usable surfaces are 55 cm x 32,5 cm (21,67 x 12,81 inch), the thickness is of 2,5 cm (0,99 inch).
The corner was made with multi-layer wood (just to keep all light but stable).
Just to save the holes into the wood and keep all easy to assemble and dismantle, I've used special threaded metal bolts, glued at the bigger panels.
And only 6 screws are needed.
Hope this example could be usefull for other users.