As MOS has said, it's well late in the season now. Get them out asap. In my opinion, little chance of hardening off issues. Covered by a bottle would be the last thing I would do now. Looking at the weather forecast, it's hot and sweetcorn weather, well apart from the lack of rain. Not even much wind. Just drape a bit of bird netting over the seedlings for a week or so, until they are big enough to not interest the birds. Water well. I've never had any sweet corn seedlings outside be nobbled by slugs, just birds when they're small, and badgers when they're just about ripe for plucking as maturing adults.
