In the System - Ad Block Reactor plugin, add the following action:

var bar = jQuery('<div/>')
 .css('background-color','#900')
 .css('text-align','center')
 .css('color','#fff')
 .css('padding','10px')
 .css('position','fixed')
 .css('bottom','0px')
 .css('left','0px')
 .css('width','100%');
bar.text('We rely on advertisements to monetize the site. Please disable your ad blocker.');
bar.appendTo(document.body);

The code above places the bar on the bottom, but you can easily change it to the top by changing .css('bottom','0px') to .css('top','0px')

The colors can also be easily changed by editing the background-color and color values.

If you used the code above unchanged, it should look like this:

A modified version of this is running on https://gdpr.richeyweb.com