My approach has varied from taking a Sainsbury's bag for life, putting all my luggage into it and taking that on board, to (most recently) buying a folding "holdall" from Ikea (£4) and dumping the stuff into that. Both times the bag fitted into the allocated space frame thingy at check in with room to spare.
This is the Ikea bag, although mine is black: https://www.ikea.com/gb/en/products/ikea-family-products/travel/knalla-sport-bag-black-white-art-50330507/
My approach has varied from taking a Sainsbury's bag for life, putting all my luggage into it and taking that on board, to (most recently) buying a folding "holdall" from Ikea (£4) and dumping the stuff into that. Both times the bag fitted into the allocated space frame thingy at check in with room to spare.
This is the Ikea bag, although mine is black: https://www.ikea.com/gb/en/products/ikea-family-products/travel/knalla-sport-bag-black-white-art-50330507/