How to call Random.Range from System.Timers.Timer handler?

So Random.Range can only be called from the Main thread.

What would be the solution then for the following where I need a random variable within a System.Timers.Timer handler?

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class FooCounter : MonoBehaviour {
private  int _randomFoo;
  private System.Timers.Timer _t = new System.Timers.Timer();
  // Use this for initialization
  void Start()
  {
  _t.Interval = 1000;
  _t.Elapsed += TimerUpdate;
  _t.Start();
  }

  public void TimerUpdate(object sender, System.Timers.ElapsedEventArgs e)
  {
  _randomFoo = Random.Range(0, 20);
  }
}