This spacer works with the IKEA Alex unit. Starting with Mattia's model, I changed the dimension: width to 20mm and height to 4mm.

You can find two models—the first is based on the original project but with a width of 20mm instead of 1.5mm, and the second matches the Basic plate in height (4mm) to perfectly adapt to the main basic grid.

[UPDATE] Added a BasicCup spacer, I use this space to store my scissors and other small tools.