It’s been suggested that an interim government would have to demonstrate it had sufficient support, eg by a public notice signed by a majority of MP’s. Perhaps it would be sufficient if the leaders of enough opposition parties + enough Tory MPs signed a declaration of support in favour of the interim PM.
But the fact that Corbyn, Swinson, sturgeon etc haven’t got their ducks in a row yet, means it’s very unlikely that they could get to that stage quickly enough following a no confidence vote. They’re not really trying to do the ground work for an interim government because it’s stuck in the “too difficult” tray.