I was looking for something to store cables and odds and ends in and stumbled across these in Tesco (Llansamlet Extra Store)
It's shallow enough to fit under the bed, but 9L is a decent size storage box.
They were marked up as starting at £5.50 but a few reductions has now seen them hit half price at £2.75
I've done a few searches and they generally sell for £5-6 on the Internet at various sites.
They had translucent blue or pink when I was there.