This provides an alternative to the official flanking rules for D&D 5e.
A creature is flanked if it is within melee reach of two or more opponents wielding melee weapons, and at least two of those opponents are on opposite sides of the creature. An opponent can't flank a creature it can't see, and it can't flank while it is incapacitated. Melee attacks against a flanked creature gain a +1 bonus for each opponent flanking it, up to a maximum of +4.
The maximum bonus is configurable in settings.
Flanking takes into account creature size and elevation.