Outdoor service panel replacement
You can cut a hole through the back, with a nipple less than 24" long you can fill it to 60%, it's hard to tell from the photo what the fill is, in the past with exterior boxes I have punched 2-3 holes installed bushings and run my wiring from a gutter where I spliced it to some thhn/ thwn and ran that outside. In my last county the inspector felt it was safer not to splice in the gutter and to run the romex into the outside service panel, I would check with your local inspector as they may allow the romex into the service panel (2 out of 3 counties allow this to a nema 3R service panel in my area). The panel must be mounted on the dwelling so the wires directly enter the home.