You can have a bed 20 cm high or even 1 meter, in different widths and lengths, it's up to you.
I made compost from boards 25 mm thick, 1 meter wide and 1 meter long, in total heigh 600mm, from 3x200mm corner 150 plank 50gap, gaps are good for compost.
I designed different types of models for planks of different widths, the length is up to you but the thickness must be 25mm, otherwise the wood must be processed.
The side holes are 3mm and the larger ones in the middle are 4mm.
The corners are made in different heights of 100mm, 150mm and 200mm, full stackable.
The recommended material is PETG or something similar that can last in an outdoor environment, when you print it in black color and PLA it can melt in sunny days.
All sizes are in mm.

3D printing settings
Prefered material PETG, 2 or more perimeters with 20+ infil, layer high doesnt matter, but for faster print 0,2mm+
Without support.
When printing 4 or more corners in one print add brim.