Simulink and selecting specific data? (More or less triggered buffering)

I have a problem, where I want to take specific parts of the signal, and turn it into a frame. The system contains a trigger and a data signal - I want to specifically take X amount of samples following a trigger, and output that as a frame. I would call it triggered buffering.

I've tried several different ways, but the problem is often that the signal not completely aligns, so data meant for one frame, suddenly shows up in another frame.

Any solution ideas?