I rescaled the original thing to 12cm x 12cm (x/y) using Cura.
Measures after printing:
So the holes are: 3.7 cm x 4.1 cm
The hanging hole is: 2 mm diameter and 2 mm thick.
That is true for my rescaled version only - not for the original thing!
To clarify things:
I did not alter the original file or design for this. It is just resizing before printing. Nothing else.
Otherwise I had appended the alternative here.
I did not use a brim, but I should had to use one, because the edges of my model have bended up (this is why I marked "Rafts" with "Yes", so you wont do the same mistake).
I used supports (created between the floor and the top hole sides only), but it may be possible to print it without. The four supports have been removed easily, because I used a "Support Z Distance" of 0.4 mm.