Corner reinforcement

International Edition (source available on Onshape, you can export part for solidworks and others formats -> right click on the left bottom column).
[EDIT] 23.5mm version. (still with M3, M5, and mdf 10mm)

Add some rigidity to the frame.

Printed with Cura, 0.4mm noze. I already adjusted hole sizes (for example: 25.5mm for a 25mm hole). Don't use Slic3r with XY_compensation.
Source available on Onshape (need to sign in first: https://cad.onshape.com/documents/56f6af3ce4b02aa4661792df/w/b2