Cornerstone for Kossel reinforcement

__Motivation__
I found the Anycubic Kossel Linear Plus frame a bit low on stiffness. In an effort to increase this, I've designed a 'cornerstone' so that I can attach some Meccano angle profile beams.
This solution is superior to any of the printed braces I've found, since the distance from the 'rotation point' can be much bigger, hence the force on the reinforcement is lower at a given bending moment and the stiffness of the meccano beam is very high, since it's made from steel vs. plastics for other braces.

__Partslist for one _lower_ frame reinforcement__
- 1x Meccano angle profile beam of 19 cm (15 holes)
- 1x cornerstone
- 4x M4 T-nut for 2020 profile
- 3x M4*10 allen screw
- 2x M4*6 screw
- 3x M4 washer

__Partslist for one _upper_ frame reinforcement__
- 1x Meccano angle profile beam of 24 cm (19 holes) [1]
- 1x Meccano angle profile beam of 31,5 cm (25 holes) [2]
- 1x cornerstone
- 2x M4 T-nut for 2020 profile
- 3x M4*10 allen screw
- 1x M4*6 screw
- 2x M4 w