It will be difficult if not impossible to do unless they are the air-seal type (and they should be if they are below an attic) with a gasket at the ceiling trim ring.

ICAT insert
insulation contact air tight insert that goes inside of
recessed light.
youll need name brand model number. go online to mfg's website and search for ICAT.

gaskets only seal oversized cut in sheetrock at can housing
when you want to seal actual holes in housing.

we have had some success with using foam ice chests--
cut to fit tightly to attic floor caulked and sealed in
place. however you must have 4" clearance on all sides
and top.

best of luck.

I have also experienced this problem. Track lighting may be the way to go.

The flies are from dead rodents in your walls. Trust me, I’ve been through it.

So then if they are in the walls wud sealing the recessed lights help the fly problem? I kill about 10 a day and i sm sooo over it!!