Here's a 2040 shim.
i've seen these used but when i tried any i found on thingiverse they slid out whenever they werent compressed. in order to fix that problem i added a boss that should fit within the cavity on the 2040 profiles to keep them centered during and after assembly.
each shim is 2mm so calculate your machine spacing/rod lengths accordingly.
you can also lower it below the bed to get the correct size shim you wanted.