What you want is illustrated in "Marlinspike Sailor", a probable 'out
of print' book highly illustrating how to do maritime 'fancy rope
work'.

Such rope fenders are essentially an 'expanded Stevedore Knot' tied
over sufficient flattened coils (to 'bulk up' the ID of the knot) to
get to the desired diameter of the fender. In the 'fender form' of
the 'stevedore knot' the 'return line' goes down the 'center' of the
knot instead of 'outside' Another way to describe the 'Stevedore'
is simply a 'figure of eight' with 'MANY turns' around the 'outside'.

You can also use a 'hangmans knot' ( but with the inside tail over the
'first' coil so that the tail end of the knot cant pass through the
inside) ... using many 'loops' inside the 'outside coils' to get to
the bulk/diameter you desire.

:-)