I designed these very simple offsets to make a cheap storage shelf I bought on Amazon compatible with filament spools. They raise the space between the shelves by 4cm. 

Will fit most spool sizes depending on where you put the rods in the shelf (see cover image).

The shelf I bought is this from Songmics:
https://www.amazon.de/dp/B08293NKQ8?psc=1&ref=ppx_yo2ov_dt_b_product_details