Oh I know that feeling - especially the one about the fish tank, as my hubby pulled a similar stunt on me.
If he does pole up with one of the ferals, then making sure you can limit that instinct to "go to ground" is important - you want to be able to make sure that anywhere he does go for safety, is somewhere you choose to leave available to him/her, and where you can access him/her for safety's sake, should you need to be able to. If you have one room you could make available for the purpose, it would be a good way to start, as a feral will need to be socialised as much as possible - having little human contact up to now will make things a little more difficult (though not insurmountable
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One of the cat carriers may be a good thing to use as a den, if you set it up in a quiet corner.
Kitten-proof everything you can think of, then retrench and have another look, as there's always something we miss. Keep the loo seat down, washing machine and tumble dryer doors shut, cables and wires out of sight, and suitably tidied. Cooker lids down if you have one with a closable lid.
Any narrow gaps or holes or openings will need to be closed off. Both Ross, Mac and Moray all took dips in our pond, but thankfully ours isn't deep, and they were well able to get themselves out, but obviously it's something to be aware of.
Toys - plenty of things for distraction and interaction - opportunities to reward play with treats. Wand toys are particularly good fun - they exercise and replicate hunting instincts. Things they can comfortably carry in their mouths work well too, such as little stuffed mice. Mac had a little sort of long legged teddy that he adored.
If the cat is a feral, then introductions with the dog will have to be carefully monitored. Scent swapping with beloved dog toys, blankets and the like will help them get accustomed to each other, but in my experience with dog/cat introductions, it's usually been an incautious or overly curious dog who has come off worst in any of our new cat encounters (my stepdaughter's jack russell, for example - who would not be told, and found out the hard way after sticking his nose into cat business that didnt concern him. Ouch!)
How exciting for you.