I couldn't find any 3D files for plates fitting the 10mm extrusion MakerBeam brands as their 'Regular' series, so I modeled them up.
All plates are 1.5mm thick and the through holes are 3mm ID per the spec.
Included is the flat plate, the 45 degree bracket, the 60 degree bracket, the 90 degree L bracket, the 90 degree equilateral bracket, and the T bracket.
Also included are the STEP files for each and a Fusion360 project file for the lot.
Side note, I spent more time looking for the files than it actually took to model them up. Of course.
PETG is probably your best bet.
The author hasn't provided the model origin yet.