They aren't very good at climbing walls, so install a 2-foot or higher barricade that has absolutely no holes in it they could get through. Sheet metal works best.

If you have corn snakes to keep the mice and chipmunks under control you are less likely to have venomous rodent eaters like rattlers. If you can encourage some king snakes, they EAT rattlers.

Do not allow anything in the run that would attract snake food (rats, mice, birds, bugs) ... dog food, dog poop, debris, etc is all attractive to snake prey, and that is attractive to snakes (as you saw with the garter snakes.

You can purchase snake fencing (see link), or build and install your own. Here's how.

There is a plastic netting sold to keep birds from eating your fruit, like cherries, etc. Buy enough of this plastic netting to go around your dog run TWICE. Be sure the holes in the net are the smallest you can get. It needs to be cut a minimum of 16" tall. Wrap your run on the outside with the netting but bury at least 4" of it in the ground. Next, fasten another length around the INSIDE of the run. You'll have to fasten it to your posts securely. We used zip ties.

You need to make sure there are no tree branches that overhang the run. Snakes can and do climb trees and could easily drop into the run from above. The snake fence will also keep any snakes who have already taken up residence there inside the run so they will have to be manually removed.

Here is a link that might be useful: Purchase Snake Fence

I live in NM and we have rattlesnakes.. we use a product called snake scram and purchase it through Amazon. We have rock walls with rod iron gates and we sprinkle it across the opening of the gates or anyway the snake could get into the yard. We seem to be having very good luck with it it's not cheap. A 5 gallon which cost $170 will last us about 3 months. You must reapply after it rains so we don't have a lot of rain if you live somewhere where there is a lot of rain it might be too costly

These snake repellents should be called Snake Scam, not Snake Scram. There is loads of empiric evidence that they don't work to repel snakes and many of them are toxic to other animals and household pets (without bothering the snakes). Moth balls in particular are made of naphthalene or paradichlorobenzene and are very toxic and should never be put outside on the ground.

Snake Scram is simply an overpriced bucket of peanut husks sprinkled with aromatic oils like clove oil, rosemary oil, cedar oil, cinnamon oil and sulphur. Studies were snakes were exposed to this sort of stuff shows that they generally crawl right over it with impunity. Just because you haven't seen snakes since you started using it doesn't mean it works. You would have probably seen the same number of snakes without it.

Dogs are much more likely to be negatively impacted by repellents such as these than the snakes.

I also don't like the suggestion of putting up that bird netting as it is horrible stuff. Any animals that get caught in it die a horrible death as the squirm and struggle to try and get out until their skin tears or they die of exhaustion over many days. Any venomous snake caught in it would be a potential threat since as it struggles it would attract the attention of the dogs and be much more likely to bite them as it struggles to escape. If it was simply crawling through the area unimpeded, it would be much less likely to bite them and they would be less likely to investigate it.

In reality the best way to keep snakes away from your yard/pets is to remove the conditions that attract them. These conditions are the same as they would be for any animal - food, water, shelter. If you remove these attractants, you will remove the snakes without ever having to come in contact with them.

A solid barrier a few feet high (even just a foot is probably enough) will discourage any snakes from entering the dog pens. If you put a sheet of aluminum flashing around the bottom of the kennel about 1 foot high, snakes will come up against this flashing and turn left or right and go around it. Biologists use this principle to make drift fences when they are studying snakes. Snakes hit the fence (flashing), turn left or right and are led right into some form of snake trap.

I know this is a late reply, but I concur with the above. Field mice, squirrels and rats can easily chew through plastic to get into the dog run. And those that can't might get caught and suffer needlessly.

I use 3 foot high, 1/4 inch woven wire fencing that I attach to my chain link fence. That stops the big snakes. Word of warning, some baby snakes are small enough to fit through the wire--especially if you need to connect segments of wire together. So adding the flashing works beautifully.