If the trailing edge material is the proper thickness, taper, etc. but just to long, you can easily correct this. (1)Cut to the proper length. (2) Cut notches in the proper locations. (3) Glue the corrected TE in place. I can't see your plans while I write this, but the trailing edge is probably sheeted so all this will be covered by the balsa sheeting. Additionally, the balsa sheeting will make up for any loss of contact if one of the notches for a rib is to large. If concerned about the usused notches, just fill it with a small piece of balsa and CA in place.

If the TE material is to thick, improper taper, etc. you still could use a profile view on the plan and sand it to shape and then use the steps above. If you have a good hobby shop close by, many sell pre-shaped pieces that might fit. With balsa wood, there is no such thing a mis-sized part. It is easy to make it larger or small.

Check out the before and after of the cowl below.