Method used on boats....
Generally, nothing is square on a boat.
Whenever I have scraps of 1/8" luan, I cut them in long strips 1" wide.
You can easily break them with your bare hands.
Lay a piece in place along one side that is cut/broken to the correct length. Then break another piece to fit along the next side and continue until you have all 4 pieces overlapping eachother. Use an electric glue gun and glue the pieces together. Cut a few additional strips for diagonal strenghtening and glue them in place. Now remove this mock-up and trace it out on your good piece.
The lengthiest part of this whole process is waiting the 3 minutes for the glue gun to heat up.
Retired, living and cruising full-time on my boat.
Currently on the Little Tennessee River near Knoxville