Body filler and resin, aka sludge, will make the shaping process easier but won't give the box any further structural integrity. All the strength is in the glass. Not even the resin,.. the resin is just the bonding agent.
If you want the smoothing process to be super easy, try to stretch the clothe over the frame in one piece, soak with resin, let it harden then add all layers of fiberglass on the inside. Keep adding layers till theres no flex at all when you push against a single point with your thumb. At most you might need a super light skim coat on top of the top layer to smooth it all out.