As an alternative i could always mount the sub panel on the adjacent wall so that it s facing 90 degrees to the main panel.
Mounting a sub panel to a concrete basement wall.
Very low water table area.
Normal placement of your new sub panel will be down in the basement directly beside the existing electric panel.
Then remove the tape.
If the back of the panel was flat i d definitely use plywood.
Basement is always dry no sump pumps no moisture coming through foundation wall.
Since the sub panel is only 14 wide i will still have at least 3 of space between it and the main panel as well as between it and the adjacent wall.
Only 1 8 or so but it keeps the moisture out.
I usually use tapcons.
If it s my choice i ll mount a panel directly to a concrete or block wall.
Just a drill and a piece of wood will.
Tape a one foot by one foot square of plastic sheeting plastic kitchen wrap will do to the concrete wall in your basement with duct tape and leave it in place for 24 hours.
This just makes sense because you connecting the sub panel directly into the existing panel.
As stated above most if not all panels have dimples in the back of the can to provide a bit of space from the wall.