I've always just disregarded the little hose nipple in this style sponge bubbler filter and just dropped an air line with an air stone down in them. Problem is, you have to swap out your air stones every few months or they clog, back pressure your air pumps too much and ruin your air pumps.
Best bang for your buck is eBay sponge bubbler filters. No air stone needed for these. You can get them for a few dollars each mailed to your door: