The rift spawns should become stunned at some point (I have read at around 10% HP), so that they can be captured by using the "Chest of Containment Coffers".
When using the cantation, only one or two rift spawns should spawn, depending on how many rifts you are in range of.
I had to aimlessly walk around the inn using it before they would spawn.
There are no visible rifts to indicate where you should use the cantation at.
Even if the coffer is down and in the world when I kill the spawn, it does not capture it. I have also tried to place the coffer first before summoning the spawn, as well as placing it midway through the fight.
The rift spawns are not capturable with the other quest item, "Chest of Containment Coffers" the rift spawns never get "stunned" and are thus never capturable.
When using the quest item "Cantation of Manifestation", all rift spawns summon at the same time, making the quest extremely difficult to do at-level.