Had one of my clowns 'go over the wall' the other day.
Mine is a corner overflow, so It may have more room in it.. I just stuck a large netdown in there (which he avoided to one side, then got trapped in) and reeled him in.
He was happily nibbling on the funk of sponges/pods
/etc that grows in the overflow..
Often times wonder if I should clean it out... but I figure it's a decent nutrient export on some level.
As for the lid.. I think that the fish actually more go through the area around the return pipe (in mine anyhow). So even if there is a lid, make sure it comes down and sort of blocks that gap too.. and be sure to leave enough clearance for the air tube.. as far as gas exchange, you could simply have holes in the top and that should be ok (the whole top of my tank is open, so I don't worry about covering up such a small area).