Tim Lamb wrote:

Any sources for *slot in* file drawers?

No, I did look for something that could be slung below the desk, but
ended up a single-width and a double-width version of these, all in
smokey black plastic and removed the castors.

https://www.reallyusefulproducts.co.uk/uk/html/onlineshop/rub/tTower6x4l.php