Another thought is fit. You need to get a pretty tight fit, if you want it to fit in-between the spars & ribs.
If you glue to the sides of the spars, the shear webs can be as tall as you want, and any shape. You jsut need to sand to shape once glued (and you'll typically do a sanding pass at some point...) But, the width needs to be right, to fit between the ribs..