I found purpose-made proofing boxes very expensive for what they were; I just bought a couple of the largest sistema food storage boxes they had at my local supermarket, which can take 4-6 dough balls each, and are stackable. (Edit: think it was these: https://www.nisbets.co.uk/sistema-klip-it-storage-container-7ltr/s488 )
I found purpose-made proofing boxes very expensive for what they were; I just bought a couple of the largest sistema food storage boxes they had at my local supermarket, which can take 4-6 dough balls each, and are stackable. (Edit: think it was these: https://www.nisbets.co.uk/sistema-klip-it-storage-container-7ltr/s488 )