We've build the setup below. Caught and transfered a witch to the left holding area. Placed a snow golem in the right holding area.
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The idea was that they would attack each other and that the cobwebs in the middle would catch the thrown items. This was a new feature introduced in snapshot 15w49a.

Unfortunately Mojang had removed this feature one snapshot later in 15w49b. So I guess we went through all that trouble for nothing

Minecraft Cobweb wiki page.
15w49a Cobwebs now catch ender pearls, snowballs, eggs, bottles o' enchanting, and thrown potions as the corresponding dropped items.
15w49b Cobwebs no longer catch thrown projectiles as dropped items.