There isn't quite the space, here in our new property, compared to the previous place so I'm being creative.
Over the years I have found that one of my best assets for protecting Monarch caterpillars has been a cheap, nylon mosquito net. I hang the net from a high point and place pots full of growing swan plants (milkweed to those of you in other countries) under it. This keeps the caterpillars safe from the killer Asian Paper Wasps, and other flying insects that might damage/eat/kill the eggs and caterpillars.
Unfortunately this won't keep spiders, ants, earwigs and the likes, out, however they can be taken out as you see them inside the enclosure. I don't seem to get that many, especially if the base is on concrete.
I have seen wasps cling onto a net and kill a caterpillar that was busily eating a leaf against the net. The wasps suck out the caterpillar's insides and leave the skin.
As you can see, I weigh down the bottom edge of the mosquito net with pieces of old concrete, rocks or bricks - seems to work alright and these are easy to lift out when I need to make adjustments to the net or get inside it. I clip the front opening shut, using ordinary clip-type clothes pegs. They work just fine and it's easy to unclip them to get into the net when I need to.
The fine nylon mosquito net is quite strong enough for the task and I have had this particular green net for 4 years. They are cheap enough to replace, when needed. Any rips or tears are easily repaired. You need to do this quickly, if your net gets torn, otherwise wasps might get in.
I have not put all my swan plants under the net. I still have some growing in the garden and a few others growing in pots outdoors. These are where the Monarch butterflies lay their eggs and I will collect the eggs as I see them.
I place the Monarch eggs into a flat plastic container and will hatch them indoors. I have a damp paper towel in the base of the container and keep that damp so the leaf pieces don't fully dry out. Once the tiny caterpillars emerge, I will put them out on the plants under the net.