Tripp Trapp adapter for IKEA TROFAST Mesh storage box.
I tested with the french product (https://www.ikea.com/fr/fr/p/trofast-boite-de-rangement-en-filet-gris-fonce-60518453) but it should work with one from any country. The dimensions of the box are 42x30x10 cm.
I advise you to tighten the screws of the chair to insert the basket.