this is exactly what i do for underground supply, single meter and side by panel
All Treated 4x4 and either 2x6, or larger, depending on various panels hole spacings
i also use a 2x4 for the bottom crosspiece/brace for any piping
#9 x 3" deck screws as required and #10 x 1.25" sheet metal screws to hold the panels
done it for years and no complaints or call backs
and i know some of them would, if they needed to, they use me exclusively
i have also had a chance to look at some of them years later, yep still good
