Today I completed the installation of the styrofoam insulation between the spars. I used 1 1/2 inch think styrofoam that comes with plastic laminated to both sides. I glued it with contact cement. The foam sheet is very rigid and will not conform to the curved contour of the cabin unless it is kerfed. I cut kerfs every inch and a half with my table saw. The kerfs are about 3/4 of the thickness of the sheet. You apply the contact cement to the surface of the insulation and to the wood surface and allow it to dry to a state where it feels is about like the gummed surface of a 3M sticky note. At that point pressing the glued surfaces together bonds them. Contact cement dissolves polystyrene foam so you apply it only to the plastic covering of the foam sheets and try to keep it out of the kerfs and off the edges of the foam.
Here's what the kerfed sheet looks like after I have bonded it to the surface.
Ta da... Insulation has been applied in all the spaces between the spars. Next step is to get all of those electrical wires into channels and then I can put the outer skin on the trailer.