I would think that snakes are more active during the day, being cold blooded creatures. So keeping them out of an open coop is a challenge. You can try killing them all as you find them, they like to curl up under warm places at night, which would mak finding them fairly easy. Baiting traps with food and heat would probably catch some snakes. But I would say the best way to get rid of them is to get a couple cats. Cats can't take down 6 foot snakes, but can and will kill any baby snakes they find, which means a lot less big snakes coming for your chickens. My outside cats don't bother my chickens at all, not even the 7 week old pullets running around my yard. But I wouldn't trust just any cat with pullets.
Keeping the coop secure at night is still very important, coons can climb and will break into coops and slaughter all the birds within. I believe possums will too. Rats are also notorious egg theifs. All of them are more active at night.
