[pnosko]

I've been getting emails every few months about this modification, and Richard even posted it on his website. I suspect I'm getting the emails from there. I've emailed Richard, asking him to remove my email link. I no longer recommend my modification. I now use mgk65's method (see his post above) as it is superior to mine. Here it is in more detail, with ever so slight changes. I see a similar method on Richard's webpage.

Get yourself a few feet of the blue silicone airline tubing (it is nice and soft) and an air value like you'd typically use for controlling an air pump.

Drill a hole in the top of your end cap very slightly smaller in diameter than the tubing. Insert one end of the tubing into this hole. Turn the endcap so that "my" holes on the side are not
aligned (effectively closing them off). Make sure the tubing doesn't go too deep into the hole; maybe just a 1/8" deeper than the hole. This soft tubing will seal the hole. Use the valve on the other end to super-fine adjust the airflow until the water level
remains constant.

You'll never have to worry about the water level submerging the hole. And in my experience, I only need to remove and clean the tubing and air hole every 3-4 months when the "sucking" sound returns.