A set of gridfinity holders for the Snapmaker U1 Spool Adapter by Post-Apocalyptic DIY:

These all take up 2x2 Gridfinity spots, but the spool adapters themselves reach further still. However, when angled properly, multiple spool adapters can be placed next to each other without leaving slots empty.

I included 3 variants:

v1: standard gridfinity cutout bin, can be stacked when empty
v2: reduced design that only has the bare necessities to hold the spool adapter and fill all 4 gridfinity slots
v3: minimal design that uses the minimal amount of filament and only sits on the cross of the gridfinity slots