Just be honest about it. You're not willing to take the box room unless you're allocated it fairly and randomly. If they moan just stand your ground, this is going to be your bedroom for a year (maybe two), you need to be happy with the allocation or you're going to resent your flatmates. They might be awkward about it but remember it's just cos they feel the same as you that they don't want the box room. You don't owe them taking it. Just be clear it's not happening and say you have two options - adjust the rents higher/lower until someone agrees to take it, or pull names out of a hat.