I put a few holes in my sub-panel to do the same ... big enough to pass through a 28 pin round connector so I wouldn't permanent cable captured in the sub-panel. Many here have hacked up the center section with holes to let a long nav radio poke back, with no issues.
On consideration would be to put adel clamps attached to the bottom, 90 deg flange, to make removal easier in the future.
Grommet or bushing? What do you have in your box? I have a wide selection of bushings, so I went that way. Either seems fine.
N430WB RV-7 @KHAF, Flying 200+ hrs
VAF 2023 donation happily made