This might be useful:

King, in _Electromagnetic Engineering_, Vol. 1, says (pp. 467-8):

". . .the problem may be analyzed for wires sufficiently far apart and
so small in radius that rotational symmetry in the interior and on the
surface of each conductor is a good approximation, and then generalized
to closely spaced conductors of large radius simply by writing

ae= a * sqrt(1 - (2a/b)^2)
be = b/2 * (1 + sqrt(1 - (2a/b)^2))

instead of a and b in formulas for the external impedance."

a is the wire radius, b the center-center spacing, ae is the radius to
be substituted and b is the spacing to be substituted. He's saying that
you can account for proximity effect just by substituting equivalent
wires of smaller diameter and closer spacing which have idealized evenly
distributed current around the outside.

He introduces the equation for ae earlier without proof, but references
a 1921 paper in an obscure publication, as a way to adjust internal
inductance for the proximity effect. The method is much simpler than the
approximate equations for added loss due to proximity effect I've come
across, so I suspect it's an approximation, but I don't know the
applicability limits. But you might try incorporating it and see how it
compares with measurements and with calculations based on the more
complicated equations.

I see that when the wires are just touching (b = 2a), you end up
substituting wires of radius zero, spaced at half the actual distance. I
suppose that's reasonable.

Roy Lewallen, W7EL