Buy some FOOD-GRADE diatomaceous earth. You can buy it easily online by running a search. Diatomaceous earth is basically a kind of "sand" with jagged edges. It pierces the outer skins of creepy-crawlies (slugs, snails, cockroaches, ants) but it harmless to humans and harmless to pets.
You just sprinkle a handful around on the ground around the feeding bowl. Just a very very light dusting works fine, and you can put it on concrete or straight on the ground. The slugs get about 0.5 cm into the diatomaceous earth and freak out and immediately turn back. You have to reapply it when it rains, as it gets washed away.
Many people with pets sprinkle this in their houses along the walls/in corners to keep bugs out of the house. It's one of the few safe ways to get cockroaches without also potentially poisoning your pets.
BTW some/many people add it to their pets' food to help their pets get rid of various intestinal bugs/worms they might have. You can of course add it to the food you put out for the strays if it's wet food, to help your strays clean out their guts.
here is a link explaining what diatomaceous earth is and how it is used, including amounts to feed to eliminate intestinal worms:
DO NOT buy the stuff made for swimming pools, it will kill your cats. It has to be "food grade".