They'll probably be able to bounce out of the tub in a couple of weeks, or worse, into the heat lamp. I strongly suggest covering it in advance with something like hardware cloth, and have the lamp outside that.
Other option: since the coop is already there (and past the quarantine period) you could just brood the chicks in the coop. That'll give them more space, and home them to the coop without any extra work.
Other option: since the coop is already there (and past the quarantine period) you could just brood the chicks in the coop. That'll give them more space, and home them to the coop without any extra work.