9cm diagonal brace - 5.5mm screw holes - 17mm

Simple diagonal brace - 9cm for Tevo Tarantula Y-frame - has mounting holes to secure this to a base plate. I guess you could use it for other corners as well.

I also have a 15cm version of this - https://www.thingiverse.com/thing:2626456

17mm web, 5mm flange, 5.5mm bolt holes (fits M4 with washers or M5)