The structure should be list.add(new ResourceRef(buffer, offset, "Trap", "PRO"));
list.add(new SectionOffset(buffer, offset + 8, "Effects list offset", null));
list.add(new DecNumber(buffer, offset + 12, 2, "Effects list size"));
list.add(new Unknown(buffer, offset + 14, 2, "Projectile"));
list.add(new DecNumber(buffer, offset + 16, 2, "Explosion frequency (frames)"));
list.add(new DecNumber(buffer, offset + 18, 2, "Duration"));
list.add(new DecNumber(buffer, offset + 20, 2, "Location: X"));
list.add(new DecNumber(buffer, offset + 22, 2, "Location: Y"));
list.add(new DecNumber(buffer, offset + 24, 2, "Location: Z"));
list.add(new DecNumber(buffer, offset + 26, 2, "Target"));
Target is so the projectile works with targeting friends/enemies (since it operates relative to the source). Z is height, frequency should be explosion length (in frames), and duration is number of explosions (these labels are set up to match NI's PRO handler).
#24 should be fine for travel_trigger. It won't break any existing use.