You are very right to be concerned about "introducing a twist". You need to avoid this.

If you afix straight balsa sticks at/near each wingtip, you will be able to "eyeball" these so as to ensure that each wingtip is at the same relative incidence. These will be especially easy if your wing has a flat-bottomed section. Lots of time will be spent in setting up your bench for this job, prior to the application of glue.

How you achieve the desired outcome depends upon so many different factors. You'll make it work, somehow.