Is it OK to have a gap at the SEC clamped connection into a panel cabinet? I could stick 2 pens into the opening shown below.
No it is not ok to use the wrong clamp.
312.5 Cabinets, Cutout Boxes, and Meter Socket Enclosures.
Conductors entering enclosures within the scope of
this article shall be protected from abrasion and shall comply
with 312.5(A) through (C).
(A) Openings to Be Closed. Openings through which conductors
enter shall be adequately closed.
314.17 Conductors Entering Boxes, Conduit Bodies, or
Fittings. Conductors entering boxes, conduit bodies, or fittings
shall be protected from abrasion and shall comply
with 314.17(A) through (D).
(A) Openings to Be Closed. Openings through which conductors
enter shall be adequately closed.
110.12(A) Unused Openings. Unused openings, other than those
intended for the operation of equipment, those intended for
mounting purposes, or those permitted as part of the design
for listed equipment, shall be closed to afford protection substantially
equivalent to the wall of the equipment. Where metallic
plugs or plates are used with nonmetallic enclosures,
they shall be recessed at least 6 mm (1⁄4 in.) from the outer
surface of the enclosure.