Had found and kept a random larva, and realized a few days later that it was pupating...really wish I had taken photos throughout the process... a few days ago I discovered a white beetle in the capsule! And here he is all growned up!
This beetle has no prominent ridges on its elytra, unlike nyctoporus. Maybe blapstinus, based on the size. Amici, could you take another picture, perhaps from above the beetle?