Electrical panel question

All I have to offer is to get or borrow a knockout punch set. drill a hole centered where you want the pipe or nipple to come in, then use the punch set to make a hole the size you need. OR possibly using the "guts" of a new panel in the existing metal enclosure

To anybody else, if you have a Zinsco or Federal Pacific (FPE) panel, get it replaced before your place burns down. THEY ARE FIRE OR SHOCK HAZARDS. Google images of those brands, because there are other companies that bought them and rebranded them. (GTE Challenger or GTE-Sylvania)
zinsco fpe fire hazard - Google Search

Sorry, just re-read OP, and see the combo meter/panel. I'd get 2 separate units. It sucks because then its a much bigger project.