I always need more storage with my kids, and this Habitat Pack of 4 Canvas Boxes is a bargain over at Argos. It's just £9, making each individual box only £2.25, and you can choose from 3 colours.
There's a choice of grey, pink or blue boxes at this price, so you can match them to your décor. They will fit lots of modular storage units, though if you're using them for inserts you'll want to check the dimensions before you order just to be on the safe side. Each one measures 28cm square, with a handle at the front so they can be used as drawers. They can also be used just on their own as extra storage.
They are made with sturdy board and fabric, and come folded flat, ready to just pop open for use. When they're no longer needed you can just fold them flat again ready to be used the next time.
Collect from store is free of charge, or home delivery costs £3.95.