After looking at the instructions the only thing I can think of is to cut peices 25 and 26. then refit them top to bottom like a sandwich. I would cut the piece into three. Cut the top and bottom off the hinge. So when you are done you will have a top, bottom and hinge piece. Then fit the top and bottom together with some masking tape. Place the hinge back into position with the brake closed and hold to gether with masing tape. Then fit it to the wing. If this works you will have the same surface the MFG designed to fit in that spot and it should fit well. This will take a while and is tedious but I believe will provide a very nice finished product. When sanding do a little at a time check fit a little more check fit etc. When you check the fit check it for how the top and bottom fit together and how well the leading edge fits to the top and bottom of the wing. You may have to glue the top and bottom to the wing seperately since these parts were not designed to fit the model in this way.
Once dry, sanded and paintedy, fit with masking tape to the wing and use CA to tack glue it in place, take the tape off and check for fit. If all looks good make it permanent if not cut the glue spots and re-fit.
Good luck! Would love to see pictures when it's done.