Used this technique on the fin can of my current 75mm airframe build and noticed about a 1mm bulge around the airframe at the fin slots.

This is LOC paper tubing, so it's beefy, but I don't recommend two-part-foam unless it's a fiberglass airframe.

You could also take the bulge as an oblique strategies prompt and lean into it.

http://jcrocket.com/foam-fincan.shtml

#rockery #tw #ObliqueStrategies