Your only options unfortunately are to utilize either the 2004 offset knuckle or the 1019 offset knuckle. The 2004 knuckle will fit in a 2004 or 1015 box, while the 1019 knuckle will fit in a 1019 arm (of which the coupler box can be removed and body-mounted) or 1016 box. The 2004 and 1019 box tops are half the thickness of their 1015 and 1016 counterparts. If you mill the quarter circle cavity in the bottom of the car around the coupler mounting hole as clearance for the MTL centering pin, you can trim the mounting lips on a 1015 coupler box and screw the box bottom directly to the underside of the floor, eliminating the thickness of the box lid.
There might be an MTL coupler in one of the vintage conversion kits that is of similar design to the 1021, but you would have to explore with them what remains available as many of those conversions are for long-discontinued (or re-engineered) motive power.