The first step I needed to take was making my wing straight and square on the end. This should have been done by shaping the spars accordingly, but I wasn't smart enough to think about that at the time. As a result, the ends of my wings are pretty wavy.
In order to fix this, I started by cutting an oversized airfoil from a piece of 2" foam, and cut it into a bunch of sections that would follow the angles on the wing endplate.
Next I laid a framing square along my front spar, which is perpendicular to the aircraft's centerline, and used that to draw a line on the top and bottom of the foam 1" outboard of the endplate. After the lines were drawn, I laid some 2" aluminum foil HVAC tape along the lines all the way around the foam.
Once that was lined up and pressed on really well, I fired up my hot wire foam cutter and used the tape as a template. When I was finished, I had a really nice straight, square, flat end plate to glue my wing tip onto.
I'll be extending my wing 15" outboard of the 3/32" plywood endplate, in order to try and squeeze a little better glide rate out of the plane. From what I've been reading, it doesn't seem to have much of a speed penalty, and a number of folks have extended their wing 18" or more this way without adding to the length of the spar. Since the foam I used to fix the wavy endplate issue is 1" outboard of the main spar, I'll add 14" of foam by cutting 7 pieces of 2" stuff and gluing it all together. It's cut to a slightly large airfoil shape to help me get it to the final shape faster. I'm using 3M spray adhesive so that my hot wire foam cutter will be able to slice through it once it's ready, although I'm slightly skeptical about how well that'll hold the foam for sanding.
Continued in Wing Tips (Part 2)