Hello, and welcome to WAB!!
The ventilated box with a small piece of cloth is ideal for tonight. You can put water in a plastic pop lid or similar (not too deep). Tomorrow, try to contact the Bat Conservation trust
Need Help with a Bat? - Bat Conservation Trust
who will be able to point you in the direction of a local bat worker.
If the RSPCA turn up for the bat, please ask them what they intend to do with it. Most people have zero knowledge of bat care. A local bat worker will be in a better position to care for it.
If it is at all injured by the cat it will probably need a short antibiotic course. Not all antibiotics are safe for use with bats. it will also need feeding - which is a delicate operation to say the least. But that can wait until tomorrow. For now it needs to be secured, and offered the water... They can get out of the smallest gaps, so make sure the box is secure and that the cat is not near to it!
Please let us know how things go tomorrow.
Jan