you are looking at exactly 25 cm for each unit (as snow peak seems to call it) so a two unit as you have should be 50cm no wider
google says 50 cm is 19.685 inches ?
(I am guilty of using the 10 inches as its easy to say for non metric folks per space)
if you built it 20 inches and say I built my own 20 inch unit custom very good chance it would not fit !!!!
I ran into that when building my stuff to test for my BBQ and stuff
so found 25cm every space every thing worked and also forgot which way it was ? but the 2 space unit a single item could drop in say with almost no wiggle room (meaning going all the way to the black ends) use your insert as example but if you tried to drop two of those into a 4 space they would not fit !!!
I did my own cutting boards for 2 space units that were a tight fit and could not fit two of them in my 4 space
the reason is those sliver bars are the width you want so the 2 space is 50 the 4 space is 100
so a 4 space total is 109cm and a 2 space is 59cm
when I built some custom length to fit tight two space units I could not fit two of them in the 4 space
the black ends give you about 9 mm wiggle room maybe a touch more on a IGT maybe ten ? would have to measure again been a while since I been doing custom fittings
hope that helps you out
and buy the way love that scottle