That 4 1/8 dimension seems to be the norm and the smallest thickness. Looking at other places with similar offerings, even lower capacity units are the same. Larger capacity commercial units are 4 3/8 to almost 5.
There was one poster who did a nice job of modifying his hold down by cutting the sides. He either replaced the side or just welded in small pieces. Ground welds so repair was all but invisible. Don't remember any more details or how he finished it off though--paint, powder coat or plating or combination thereof.