Snake biologist here. Snake repellants are not effective, nor are various other oils/materials/foods spread around the area you want protected. Rat snakes, in particular, specialize in eggs/small birds during this time of year. If you would like to keep snakes out of your pens, hardware cloth is hands-down the best bet, I find. Adding various birds/other animals to your yard might help reduce activity, but is incredibly unreliable. Plastic/other netting not rated to be bird-safe can result in entanglement of your personal critters. If hardware cloth is too expensive, perhaps consider stapling window screen to your pen, although large/determined enough snakes might be able to push open old screens.
Honestly, if your coop cannot keep out medium-sized rat snakes, you're not going to be able to keep out rodents either, which can cause more problems. Rats love bird food, eggs, and warm/dry homes to nest in. It's my view that you should be rat/snake-proofing any coop you build.
Now to my personal view on managing snakes: I would ask you consider not killing them. The presence of snakes is natural and means you live in a healthy ecosystem. Thoughtful coop construction should prevent significant snake activity and is definitely one of my top priorities for the safety/security of my birds against all predators. I would never shoot a robin for pulling worms out of my compost, a sparrow for eating my tomatoes from my garden, or a deer for grazing on my lawn. Venomous snakes can be removed by local snake removal services, who will oftentimes help you prevent snakes from returning. Generally, their advice is to remove sources of food or areas where the snakes like to snuggle up in. We can't really achieve those things easily, so our best bet is to be conscious of our coop design.
Ok, time to step off the soap box. Anyone can feel free to message me with questions about snakes or reptiles! I study them for a living and love helping people out.