Mesh

I mainly use fly screen intended for doors or windows. It's relatively cheap and available in almost every hardware store. Also with most products, you can choose between black and white, which I find very useful to adapt to different purposes. The mesh size has to be a millimeter or under, at least in my experience. This is small enough to prevent most caterpillars, even in their first instars from escaping (maybe some really small, freshly hatched fit through). Provided there are no gaps in the mesh, most parasitic wasps and flies won't get through. Some very small like Chalcidoidea, for example, are able to even get through these small holes, but you can't really prevent that anyways. I believe there are certain wire meshes that are available in under 1 millimeter, but they are very expensive. I also don't like wire mesh, because it seems to be not as flexible, but that's just my preference, maybe others will find it to be better.

Greetings Dennis