Trev,
I used closed cell pipe insulation on mine. It was tight as you say. You might consider putting the expanded foam between the volcano wrap and the pipes on the side to try and keep the pipes from touching or at least filling the gap formed there. If worried about expansion, tie some cardboard along the sides. spray the foam and trim the stuff that sticks out, then rewrap the pipes? Just some thoughts. If that is too messy then put the closed cell under the wrap along the sides. Every bit helps.
Bill