Go get a long straight edge ruler from the local home improvement store, or some angle aluminum (or steel).  Using a flat building surface and some weights, pins, or clamps build against the straight edge.  If you put a coat of car wax onto these tools regularly glue will never stick.

If you have to put an angle into the joint for tapering (or dihedral) then use 2 straight edges fastened at the proper angle.  I draw directly onto the building surface to construct the desired angle.

Some plastic, wax paper, or Glad Press-N-Seal will keep the work from gluing to the building surface.

sorry, no pictures.

Phil

I now use parchment paper to keep from gluing stuff to surfaces.   

a builders Spirit Level , if it hasnt been used as a crow bar , can be usefull for laminateing on .



Done Spruce capped I beam spars on these . One with vertical grain 3/32 shear web & 1/16 ply intermittent spacers/ braces , 1/4 Sq. spuce top & bottom.

The Other a 1/4 sheet spar with ply joiners , bellcrank cut out , & 3/8 x 1/8 spruce top & bottom . Tapered . Taped and packed / Measured central on the flat of the level .

Just joinig / laminating untapered strips youd use a corner as an edge to keep em straight . Really Mean & you can clamp to it . Use a pine batten or suchlike .
Clamps direct on the balsa leave clampmarks & youre aiming for the pressue even along the join .

add = Scrap Barrel at the Aluminum Joinery Co in N.Z. is a good source of long straight angles , rectangles & tubes. and not so long ones . ribbed angles for clamping , I guess . Usually FREE .