Printed with PLA.
Can easily support 10 kg (I tested with 15 kg without braking).
All screws are hidden.
I used screws with 3.5x50 mm.
Shelf size:
Thin - 221mm (W) x 220 mm (D) x 15 mm (H)
Large - 210 mm (W) x 250 mm (D) x 30 mm (H)
Tips to print: When printing the shelf (no need for the support), disable Auxiliar Fan for the last 4 layers.