Securing sparrow nest site under the roof

Hello and apologies if this topic has already been covered and I missed it. This spring we've had 2 emergencies related to the sparrows nest in our roof. They've been nesting there for years and everything was fine but this year things went wrong. In May we rescued 3 fledglings who fell down a cavity between the wall and window. It was relatively easy to get them out but we couldn't reach the nest to put them back in so we took them to the wonderful Wildlife Lodge. Tow out of three made it and they were released about 2 weeks ago :). Then last weekend another chick (this time a very small one, from a new brood) fell out and got into a very awkward place. It was a major rescue operation to get him out from under the floor in the loft (no idea how he managed to get this far from the nest!). He's also safely at the Wildlife Lodge. Obviously we need to secure this nest site as soon as we can, so nothing like this happens again (imagine if the birds fell somewhere we couldn't get them out from!). Just wondered if anyone had done it successfully and could advise the best way to do it. They are still nesting so I know we need to wait until at least September and keep our fingers crossed no one else falls out. I don't want to block it completely if at all possible. By the way, we put up a terrace box this year but blue tits got there first and the other two "rooms" were left unoccupied. No one fancied a single box lower down...