RE: using expanding foam in built up wings
You can use those long mini control rod plastic tubes they use for small electric park flyer RC airplanes as tube extensions to pinpoint the 1/2 second foam squirt exactly on the bullseye area you are targeting with the foam squirt. Seriously, experiment first with crashed balsa wings retrieved from the model field trash cans. I see them all the time, especially on a busy weekend. No harm, no foul.
larry