I need to fill a tall narrow gap in a plaster wall as shown in the image below. My question is: what is the best way to repair this? I have cut some ~1 pieces of drywall to fill the gap then tape the gaps from the drywall to the plaster however, I have been having a hard time cutting such thin pieces of drywall without it falling apart. Is there a better way to do this? the distance is about 8' from floor to ceiling.