Did I not see on this forum, last year or so, a special nest box where the egg rolls away when laid? It didn't seem too expensive, and I believe was to discourage egg eating, but would be equally effective against crows and magpies.
On the subject of larsen traps - I'm not trying to start a big argument, but I believe they are incredibly cruel. I know corvids kill young birds, but so do domestic cats, in large quantities, and we don't go round shooting them or shutting them in traps in order to attract others.
Apparently about 70% of what crows and magpies eat is of vegetable origin, and they normally only eat young birds in spring in order to feed their own offspring. They don't, like cats, just kill them for fun.
I'm not saying that I'm happy about the death of small birds and their young, but it's nature after all. I have a family of magpies in my garden, and I feed them. They don't touch my eggs, or my chicks which wander about the garden, because they have alternative food.
Crows and magpies are very intelligent birds. To shut them in a trap must cause them incredible stress. For a start, they probably have offspring of their own that they will be desperate to get back to, knowing that otherwise the young will starve to death. Then at night, can you imagine how terrified they will be with foxes and other predators prowling round the cage. Like I say, I'm not trying to be controversial, just giving another point of view