Some suggestions...Square-D makes NEMA-12 [NOT '12K' or ‘AWK’ suffix] XOP fusible disconnect switches without knockouts. Westinghouse/C-H makes NEMA-12 breaker enclosures of similar construction. Many 5½" depth panelboard cans are available with 'blank endwalls.' The 12’s are equal or somewhat less in price compared to 3R. ‘NOKO’ NEMA-1, -3 and -3R screw-cover gutters and pull cans are out there too.

KO reducing rings are {or should be} suicide.